ipq40xx: add MikroTik cAP ac support

This adds support for the MikroTik RouterBOARD RBcAPGi-5acD2nD
(cAP ac), a  indoor dual band, dual-radio 802.11ac wireless AP, two
10/100/1000 Mbps Ethernet ports.

See https://mikrotik.com/product/cap_ac

 for more info.

Specifications:
 - SoC: Qualcomm Atheros IPQ4018
 - RAM: 128 MB
 - Storage: 16 MB NOR
 - Wireless:
   · Built-in IPQ4018 (SoC) 802.11b/g/n 2x2:2, 2.5 dBi antennae
   · Built-in IPQ4018 (SoC) 802.11a/n/ac 2x2:2, 2.5 dBi antennae
 - Ethernet: Built-in IPQ4018 (SoC, QCA8075) , 2x 1000/100/10 port,
   PoE in and passive PoE out

Unsupported:
 - PoE out

Installation:
Boot the initramfs image via TFTP and then flash the sysupgrade
image using "sysupgrade -n"

Signed-off-by: Alar Aun <alar.aun@gmail.com>