ipq40xx: add support for MikroTik SXTsq 5 ac

ipq40xx: add support for MikroTik SXTsq 5 ac

This commit adds support for the MikroTik SXTsq 5 ac (RBSXTsqG-5acD),
an outdoor 802.11ac wireless CPE with one 10/100/1000 Mbps Ethernet
port.

Specifications:
 - SoC: Qualcomm Atheros IPQ4018
 - RAM: 256 MB
 - Storage: 16 MB NOR
 - Wireless: IPQ4018 (SoC) 802.11a/n/ac 2x2:2, 16 dBi antennae
 - Ethernet: IPQ4018 (SoC) 1x 10/100/1000 port, 10-28 Vdc PoE in
 - 1x Ethernet LED (green)
 - 7x user-controllable LEDs
  · 1x power (blue)
  · 1x user (green)
  · 5x rssi (green)

Note:
 Serial UART is probably available on the board, but it has not been
 tested.

Flashing:
 Boot via TFTP the initramfs image. Then, upload a sysupgrade image
 via SSH and flash it normally. More info at the "Common procedures
 for MikroTik products" page https://openwrt.org/toh/mikrotik/common.

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