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

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

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

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)
 - 3x green LEDs (1x lan, 1x wlan, 1x user)

 See https://mikrotik.com/product/RBwAP2nD for more info.

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: David Musil <0x444d@protonmail.com>