ath79: add support for MikroTik RouterBOARD hAP ac lite

ath79: add support for MikroTik RouterBOARD hAP ac lite

The MikroTik RB952Ui-5ac2nD (sold as hAP ac lite) is an indoor 2.4Ghz
and 5GHz AP/router with a 2 dBi integrated antenna.

See https://mikrotik.com/product/RB952Ui-5ac2nD for more details.

Specifications:
 - SoC: QCA9533
 - RAM: 64MB
 - Storage: 16MB NOR
 - Wireless: QCA9533 802.11b/g/n 2x2 / QCA9887 802.11a/n/ac 2x2
 - Ethernet: AR934X switch, 5x 10/100 ports,
    10-28 V passive PoE in port 1, 500 mA PoE out on port 5
 - 6 user-controllable LEDs:
   - 1x user (green)
   - 5x port status (green)

Flashing:
 TFTP boot initramfs image and then perform sysupgrade. The "Internet"
 port (port number 1) must be used to upload the TFTP image, then
 connect to any other port to access the OpenWRT system.
 Follow common MikroTik procedure as in
 https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>