ath79: add support for RouterBOARD mAP

ath79: add support for RouterBOARD mAP

The MikroTik mAP-2nd (sold as mAP) is an indoor 2.4Ghz AP with
802.3af/at PoE input and passive PoE passthrough.

See https://mikrotik.com/product/RBmAP2nD for more details.

Specifications:
 - SoC: QCA9533
 - RAM: 64MB
 - Storage: 16MB NOR
 - Wireless: QCA9533 802.11b/g/n 2x2
 - Ethernet: 2x 10/100 ports,
    802.3af/at PoE in port 1, 500 mA passive PoE out on port 2
 - 7 user-controllable LEDs

Note: the device is a tiny AP and does not distinguish between both
ethernet ports roles, so they are both assigned to lan.
With the current setup, ETH1 is connected to eth1 and ETH2 is connected
to eth0 via the embedded switch port 2.

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. The "ETH1" port
 must be used to upload the TFTP image. Follow common MikroTik procedure
 as in https://openwrt.org/toh/mikrotik/common.

Tested-By: Andrew Powers-Holmes <aholmes@omnom.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
(cherry picked from commit e1223dbee332b89caf71850eb909104529595c31)