ath79: add support for MikroTik RouterBOARD mAP lite

ath79: add support for MikroTik RouterBOARD mAP lite

The MikroTik RouterBOARD mAPL-2nd (sold as mAP Lite) is a small
2.4 GHz 802.11b/g/n PoE-capable AP.

See https://mikrotik.com/product/RBmAPL-2nD for more info.

Specifications:
 - SoC: Qualcomm Atheros QCA9533
 - RAM: 64 MB
 - Storage: 16 MB NOR
 - Wireless: Atheros AR9531 (SoC) 802.11b/g/n 2x2:2, 1.5 dBi antenna
 - Ethernet: Atheros AR8229 (SoC), 1x 10/100 port, 802.3af/at PoE in
 - 4 user-controllable LEDs:
   · 1x power (green)
   · 1x user (green)
   · 1x lan (green)
   · 1x wlan (green)

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. Follow common
 MikroTik procedure as in https://openwrt.org/toh/mikrotik/common.

Note: following 781d4bfb397cdd12ee0151eb66c577f470e3377d
 The network setup avoids using the integrated switch and connects the
 single Ethernet port directly. This way, link speed (10/100 Mbps) is
 properly reported by eth0.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
(cherry picked from commit eb38af788180d624e5b37aa5db1fe3766b138dc8)