ath79: Add Aruba AP-175 support

This board is very similar to the Aruba AP-105, but is
outdoor-first. It is very similar to the MSR2000 (though certain
MSR2000 models have a different PHY[^1]).

A U-Boot replacement is required to install OpenWrt on these
devices[^2].

Specifications
--------------
* Device:	Aruba AP-175
* SoC:		Atheros AR7161 680 MHz MIPS
* RAM:		128MB - 2x Mira P3S12D40ETP
* Flash:	16MB MXIC MX25L12845EMI-10G (SPI-NOR)
* WiFi:		2 x DNMA-H92 Atheros AR9220-AC1A 802.11abgn
* ETH:		IC+ IP1001 Gigabit + PoE PHY
* LED:		2x int., plus 12 ext. on TCA6416 GPIO expander
* Console:	CP210X linking USB-A Port to CPU console @ 115200
* RTC:		DS1374C, with internal battery
* Temp:		LM75 temperature sensor

Factory installation:

- Needs a u-boot replacement. The process is almost identical to that
  of the AP105, except that the case is easier to open, and that you
  need to compile u-boot from a slightly different branch:
  https://github.com/Hurricos/u-boot-ap105/tree/ap175

  The instructions for performing an in-circuit reflash with an
  SPI-Flasher like a CH314A can be found on the OpenWrt Wiki
  (https://openwrt.org/toh/aruba/ap-105); in addition a detailed guide
  may be found on YouTube[^3].

- Once u-boot has been replaced, a USB-A-to-A cable may be used to
  connect your PC to the CP210X inside the AP at 115200 baud; at this
  point, the normal u-boot serial flashing procedure will work (set up
  networking; tftpboot and boot an OpenWrt initramfs; sysupgrade to
  OpenWrt proper.)

- There is no built-in functionality to revert back to stock firmware,
  because the AP-175 has been declared by the vendor[^4] end-of-life
  as of 31 Jul 2020. If for some reason you wish to return to stock
  firmware, take a backup of the 16MiB flash before flashing u-boot.

[^1]: https://github.com/shalzz/aruba-ap-310/blob/master/platform/bootloader/apboot-11n/include/configs/msr2k.h#L186

[^2]: https://github.com/Hurricos/u-boot-ap105/tree/ap175

[^3]: https://www.youtube.com/watch?v=Vof__dPiprs

[^4]: https://www.arubanetworks.com/support-services/end-of-life/#product=access-points&version=0

Signed-off-by: Martin Kennedy <hurricos@gmail.com>

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