ar71xx: add support for the MikroTik RB911-2Hn/5Hn boards

ar71xx: add support for the MikroTik RB911-2Hn/5Hn boards

The patch adds support for the MikroTik RB911-2Hn (911 Lite2)
and the RB911-5Hn (911 Lite5) boards:

  https://mikrotik.com/product/RB911-2Hn
  https://mikrotik.com/product/RB911-5Hn

The two boards are using the same hardware design, the only difference
between the two is the supported wireless band.

Specifications:
  * SoC: Atheros AR9344 (600MHz)
  * RAM: 64MiB
  * Storage: 16 MiB SPI NOR flash
  * Ethernet: 1x100M (Passive PoE in)
  * Wireless: AR9344 built-in wireless MAC, single chain
              802.11b/g/n (911-2Hn) or 802.11a/g/n (911-5Hn)

Notes:
  * Older versions of these boards might be equipped with a NAND
    flash chip instead of the SPI NOR device. Those boards are not
    supported (yet).
  * The MikroTik RB911-5HnD (911 Lite5 Dual) board also uses the
    same hardware. Support for that can be added later with little
    effort probably.

Installation:

1. Setup a DHCP/BOOTP Server with the following parameters:
   * DHCP-Option 66 (TFTP server name): pointing to a local TFTP
     server within the same subnet of the DHCP range
   * DHCP-Option 67 (Bootfile-Name): matching the initramfs filename
     of the to be booted image. The usable intramfs files are:
       - openwrt-ar71xx-mikrotik-vmlinux-initramfs.elf
       - openwrt-ar71xx-mikrotik-vmlinux-initramfs-lzma.elf
       - openwrt-ar71xx-mikrotik-rb-nor-flash-16M-initramfs-kernel.bin

2. Press the reset button on the board and keep that pressed.

3. Connect the board to your local network via its ethernet port.

4. Release the button after the LEDs on the board are turned off.
   Now the board should load and start the initramfs image from
   the TFTP server.

5. Upload the sysupgrade image to the board with scp:
     $ scp openwrt-ar71xx-mikrotik-rb-nor-flash-16M-squashfs-sysupgrade.bin root@192.168.1.1:/tmp/fw.bin

5. Log in to the running system listening on 192.168.1.1 via ssh
   as root (without password):
     $ ssh root@192.168.1.1

7. Flash the uploaded firmware file from the ssh session via the
   sysupgrade command:
     root@OpenWrt:~# sysupgrade /tmp/fw.bin

Signed-off-by: Gabor Juhos <juhosg@freemail.hu>