ipq40xx: add support for Mikrotik wAP R ac / LTE / LTE6

ipq40xx: add support for Mikrotik wAP R ac / LTE / LTE6

The Mikrotik wAP R AC is an outdoor, dual band, dual radio (802.11ac) AP
with a miniPCIe slot for a LTE modem.
The wAP R AC is similar to the wAP AC but with the miniPCIe slot.
The wAP R AC requires installing a LTE modem.
The wAP LTE and wAP LTE6 comes with a LTE modem installed.

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

Specifications:
- SoC: Qualcomm Atheros IPQ4018
- CPU: 4x ARM Cortex A7
- RAM: 128MB
- Storage: 16MB NOR flash
- Wireless:
   - Built-in IPQ4018 (SoC) 802.11b/g/n 2x2:2, internal antenna
   - Built-in IPQ4018 (SoC) 802.11a/n/ac 2x2:2, internal antenna
- Ethernet: Built-in IPQ4018 (SoC, QCA8075) , 2x 1000/100/10 ports
  one with 802.3af/at PoE in
- 1x Mini PCI-E port (USB2)

Installation:
Boot the initramfs image via TFTP, then flash the sysupgrade image using
sysupgrade. Details at https://openwrt.org/toh/mikrotik/common.

Signed-off-by: Alexander Couzens <lynxis@fe80.eu>