realtek: add support for TP-Link SG2452P v4 aka T1600G-52PS v4

This is an RTL8393-based switch with 802.3af on all 48 ports.

Specifications:
---------------
 * SoC:       Realtek RTL8393M
 * Flash:     32 MiB SPI flash
 * RAM:       256 MiB
 * Ethernet:  48x 10/100/1000 Mbps with PoE+
 * Buttons:   1x "Reset" button, 1x "Speed" button
 * UART:      1x serial header, unpopulated
 * PoE:       12x TI TPS23861 I2C PoE controller, 384W PoE budget
 * SFP:       4 SFP ports

Works:
------
  - (48) RJ-45 ethernet ports
  - Switch functions
  - Buttons
  - All LEDs on front panel except port LEDs
  - Fan monitoring and basic control

Not yet enabled:
----------------
  - PoE - ICs are not in AUTO mode, so the kernel driver is not usable
  - Port LEDs
  - SFP cages

Install via web interface:
-------------------------

Not supported at this time.

Install via serial console/tftp:
--------------------------------

The U-Boot firmware drops to a TP-Link specific "BOOTUTIL" shell at
38400 baud. There is no known way to exit out of this shell, and no
way to do anything useful.

Ideally, one would trick the bootloader into flashing the sysupgrade
image first. However, if the image exceeds 6MiB in size, it will not
work. To install OpenWRT:

Prepare a tftp server with:
 1. server address: 192.168.0.146
 2. the image as: "uImage.img"

Power on device, and stop boot by pressing any key.
Once the shell is active:
 1. Ground out the CLK (pin 16) of the ROM (U6)
 2. Select option "3. Start"
 3. Bootloader notes that "The kernel has been damaged!"
 4. Release CLK as soon as bootloader thinks image is corrupted.
 5. Bootloader enters automatic recovery -- details printed on console
 6. Watch as the bootloader flashes and boots OpenWRT.

Blind install via tftp:
-----------------------

This method works when it's not feasible to install a serial header.

Prepare a tftp server with:
 1. server address: 192.168.0.146
 2. the image as: "uImage.img"
 3. Watch network traffic (tcpdump or wireshark works)
 4. Power on the device.
 5. Wait 1-2 seconds then ground out the CLK (pin 16) of the ROM (U6)
 6. When 192.168.0.30 makes tftp requests, release pin 16
 7. Wait 2-3 minutes for device to auto-flash and boot OpenWRT

Signed-off-by: Andreas Böhler <dev@aboehler.at>

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