ath79: support for MikroTik RouterBOARD 911G-(2,5)HPnD

This is a stripped-down version of RB912UAG-(2,5)HPnD, without USB,
miniPCIe and SIM sockets.
This board has been supported in the ar71xx.

Add support based on RB912UAG board, by splitting out the common part to
.dtsi, and creating separate device tree for the stripped-down version.

Links:
* https://mikrotik.com/product/RB911G-2HPnD
* https://mikrotik.com/product/RB911G-5HPnD
* https://openwrt.org/toh/hwdata/mikrotik/mikrotik_rb911g-5hpnd

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 a/b/g/n (depending on band variant)
* LEDs: 5 general purpose LEDs (led1..led5), power LED, user LED,
  Ethernet phy LED,
* Button,
* Beeper.

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.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>

target/linux/ath79/mikrotik/base-files/lib/upgrade/platform.sh

@@ -30,6 +30,7 @@ platform_do_upgrade() {
 
 	case "$board" in
 	mikrotik,routerboard-493g|\
+	mikrotik,routerboard-911g-xhpnd|\
 	mikrotik,routerboard-912uag-2hpnd|\
 	mikrotik,routerboard-921gs-5hpacd-15s|\
 	mikrotik,routerboard-922uags-5hpacd|\