ath79: add support for MikroTik RouterBOARD wAPR-2nD (wAP R)

ath79: add support for MikroTik RouterBOARD wAPR-2nD (wAP R)

This patch adds support for the MikroTik RouterBOARD wAPR-2nD (wAP R)
router, a weatherproof 2.4 GHz access point with a miniPCI-e slot and
a SIM card slot.

Specifications:

 - SoC: Qualcomm Atheros QCA9533
 - Flash: 16 MB (SPI)
 - RAM: 64 MB
 - Ethernet: 1x 10/100 Mbps (PoE in)
 - WiFi: AR9531 2T2R 2.4 GHz (SoC)
 - miniPCI-e slot
 - 4x green LEDs (1x WiFi, 3x RSSI)
 - 1x reset button

 See https://mikrotik.com/product/RBwAPR-2nD for more details.

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

Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>