ath79: add support for MikroTik RouterBOARD 911 Lite2/Lite5

ath79: add support for MikroTik RouterBOARD 911 Lite2/Lite5

Forward-port from ar71xx target the board introduced in commit
eb9e3651dd1a (" ar71xx: add support for the MikroTik RB911-2Hn/5Hn
boards"). Citing:

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).[1]
  * The MikroTik RB911-5HnD (911 Lite5 Dual) board also uses the
    same hardware. Support for that can be added later with little
    effort probably.[2]

End of citation.

Follow intallation instruction from that commit message, using
openwrt-ath79-mikrotik-mikrotik_routerboard-911-lite-initramfs-kernel.bin
and
openwrt-ath79-mikrotik-mikrotik_routerboard-911-lite-squashfs-sysupgrade.bin
images found in ath79/mikrotik directory. Be advised that the board
accepts 10-30 V on PoE input.

Known issues
Compared to ar71xx target image, there is still small leak of current to
user LED, which makes it lit, although weaker, even if brightness is set
to 0. The cause of that is still unknown.

1. https://github.com/openwrt/openwrt/pull/3652
2. RB911-5HnD should work with this commit or with [1], depending on
   what flash topology was used.

Signed-off-by: Tomasz Maciej Nowak <tmn505@gmail.com>