ramips: add support for MikroTik RouterBOARD RBM11g

This commit adds support for the MikroTik RouterBOARD RBM11g.

=Hardware=

The RBM11g is a mt7621 based device featuring one GbE port and one
miniPCIe slot with a sim card socket and USB 2.0.

==Switch==

The single onboard Ethernet port is connected the CPU directly.
The internal switch of the mt7621 SoC is disabled.

==Flash==

The device has one spi nor flash chip. It is a 128 Mbit winbond 25Q128FVS
connected to CS0.

==PCIe==

The board features a single miniPCIe slot. It has a dedicated mini SIM
socket and a USB 2.0 port. Power to the miniPCIe slot is controlled via
GPIO9.

==USB==

There are no external USB ports.

==Power==

The board can accept both, passive PoE and external power via a 2.1 mm
barrel jack (center-positive). The input voltage range is 11-32 V.

==Serial port==

The device does have an onboard UART on an unpopulated header next to the
flash chip:

GND: pin 2
 TX: pin 7
 RX: pin 6

Settings: 115200, 8N1

See below illustration for positioning of the header.

0 = screw hole
* = some pin
T = TX  pin
R = RX  pin
G = GND pin

Pinout:
+---------------
|O
|             __
|            /  \
|            \__/
|
|
|
|               +---+
|               |RAM|
| +--+          |   |
| |**|  <- unpopulated header with UART
| |*T|          +---+
| |R*|        +--------+
| |**|        |        |
| |G*|        |  CPU   |
| +--+        |        |
|    +--+     |        |
|    |  |     +--------+
|    +--+  <- flash chip
|O
|       +-----+
|       |     |
|+--+   |     |
||  |   |     |
+---------------------

=Installation=

To install an OpenWRT image to the device two components must be built:

1. A openwrt initramfs image
2. A openwrt sysupgrade image

===initramfs & sysupgrade image===

Select target devices "Mikrotik RBM11G" in
openwrt menuconfig and build the images. This will create the images
"openwrt-ramips-mt7621-mikrotik_rbm11g-initramfs-kernel.bin" and
"openwrt-ramips-mt7621-mikrotik_rbm11g-squashfs-sysupgrade.bin" in the
output directory.

==Installing==

**Make sure to back up your RouterOS license in case you do ever want to
go back to RouterOS using "/system license output" and back up the
created license file.**

When rebooted the board will try booting via ethernet first. If your
board does not boot via ethernet automatically you will have to attach
to the serial port and set ethernet as boot device within RouterBOOT.

1. Set up a dhcp server that points the bootfile to tftp server serving
   the "openwrt-ramips-mt7621-mikrotik_rbm11g-initramfs-kernel.bin"
   initramfs image
2. Connect to ethernet port on board
3. Power on the board
4. Wait for OpenWrt to boot

Right now OpenWrt will be running with a SSH server listening. Now
OpenWrt must be flashed to the devices flash:

1. Copy "openwrt-ramips-mt7621-mikrotik_rbm11g-squashfs-sysupgrade.bin"
   to the device using scp.
2. Write openwrt to flash using "sysupgrade
   openwrt-ramips-mt7621-mikrotik_rbm11g-squashfs-sysupgrade.bin"

Once the flashing completes the board will reboot. Disconnect from the
devices ethernet port or stop the DHCP/TFTP server to prevent the device
from booting via ethernet again.
The device should now boot straight to OpenWrt.

Signed-off-by: Tobias Schramm <tobleminer@gmail.com>

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