ath79: support MikroTik RouterBOARD 911G-5HPacD

ath79: support MikroTik RouterBOARD 911G-5HPacD

The MikroTik RouterBOARD 911G-5HPacD is a stripped-down version of
RB921GS-5HPacD, removing the SFP cage.
This ports the board from ar71xx, and is based on support for
RB921GS-5HPacD.

Disable mdio1 and eth1 nodes in routerboard-92x.dtsi, then re-enable
them in devices using that, so the newly-added device has the port
disabled properly.

See https://mikrotik.com/product/RB911G-5HPacD for more info.

Specifications:
 - SoC: Qualcomm Atheros QCA9558 (720 MHz)
 - RAM: 128 MB
 - Storage: 128 MB NAND
 - Wireless: external QCA9892 802.11a/ac 2x2:2
 - Ethernet: 1x 1000/100/10 Mbps, integrated, via AR8031 PHY, passive PoE in

Working:
 - NAND storage detection
 - Ethernet
 - Wireless
 - 1x user LED (blinks during boot, sysupgrade)
 - Reset button
 - Sysupgrade

Installation:
 - Boot initramfs image via TFTP and then flash sysupgrade image

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