ipq40xx: add support for MikroTik hAP ac3

ipq40xx: add support for MikroTik hAP ac3

This adds support for the MikroTik RouterBOARD RBD53iG-5HacD2HnD
(hAP ac³), a  indoor dual band, dual-radio 802.11ac
wireless AP with external omnidirectional antennae, USB port, five
10/100/1000 Mbps Ethernet ports and PoE passthrough.

See https://mikrotik.com/product/hap_ac3 for more info.

Specifications:
 - SoC: Qualcomm Atheros IPQ4019
 - RAM: 256 MB
 - Storage: 16 MB NOR + 128 MB NAND
 - Wireless:
   · Built-in IPQ4019 (SoC) 802.11b/g/n 2x2:2, 3 dBi antennae
   · Built-in IPQ4019 (SoC) 802.11a/n/ac 2x2:2, 5.5 dBi antennae
 - Ethernet: Built-in IPQ4019 (SoC, QCA8075) , 5x 1000/100/10 port,
             passive PoE in, PoE passtrough on port 5
- 1x USB Type A port

Installation:
1. Boot the initramfs image via TFTP
2. Run "cat /proc/mtd" and look for "ubi" partition mtd device number, ex. "mtd1"
3. Use ubiformat to remove MikroTik specific UBI volumes
* Detach the UBI partition by running: "ubidetach -d 0"
* Format the partition by running: "ubiformat /dev/mtdN -y"
Replace mtdN with the correct mtd index from step 2.
3. Flash the sysupgrade image using "sysupgrade -n"

Signed-off-by: Robert Marko <robimarko@gmail.com>
Tested-by: Mark Birss <markbirss@gmail.com>
Tested-by: Michael Büchler <michael.buechler@posteo.net>
Tested-by: Alex Tomkins <tomkins@darkzone.net>