ath79: add support for MikroTik RouterBOARD 912UAG-2HPnD

ath79: add support for MikroTik RouterBOARD 912UAG-2HPnD

This board has been supported in the ar71xx.

Links:
* https://mikrotik.com/product/RB912UAG-2HPnD
* https://openwrt.org/toh/hwdata/mikrotik/mikrotik_rb912uag-2hpnd

This also supports the 5GHz flavour of the board.

Hardware:
* SoC: Atheros AR9342,
* RAM: DDR 64MB,
* SPI NOR: 64KB,
* NAND: 128MB,
* Ethernet: x1 10/100/1000 port with passive POE in,
* Wi-Fi: 802.11 b/g/n,
* PCIe,
* USB: 2.0 EHCI controller, connected to mPCIe slot and a Type-A
  port -- both can be used for LTE modem, but only one can be
  used at any time.
* LEDs: 5 general purpose LEDs (led1..led5), power LED, user LED,
  Ethernet phy LED,
* Button,
* Beeper.

Not working:
* Button: it shares gpio line 15 with NAND ALE and NAND IO7,
  and current drivers doesn't easily support this configuration,
* Beeper: it is connected to bit 5 of a serial shift register
  (tested with sysfs led trigger timer). But kmod-gpio-beeper
  doesn't work -- we left this as is for now.

Flashing:
* Use the RouterBOARD Reset button to enable TFTP netboot,
boot kernel and initramfs and then perform sysupgrade.
* From ar71xx OpenWrt firmware run:
  $ sysupgrade -F /tmp/<sysupgrade.bin>
For more info see: https://openwrt.org/toh/mikrotik/common.

Co-Developed-by: Koen Vandeputte <koen.vandeputte@citymesh.com>
Reviewed-by: Sergey Ryazanov <ryazanov.s.a@gmail.com>
Signed-off-by: Denis Kalashnikov <denis281089@gmail.com>