ipq40xx: add support for MikroTik hAP ac3 LTE6 kit

ipq40xx: add support for MikroTik hAP ac3 LTE6 kit

This adds support for the MikroTik RouterBOARD RBD53GR-5HacD2HnD
(hAP ac³ LTE6 kit), an  indoor dual band, dual-radio 802.11ac
wireless AP with built-in Mini PCI-E LTE modem, one USB port, five
10/100/1000 Mbps Ethernet ports.

See https://mikrotik.com/product/hap_ac3_lte6_kit for more info.

Specifications:
 - SoC: Qualcomm Atheros IPQ4019
 - RAM: 256 MB
 - Storage: 16 MB NOR
 - Wireless:
   · Built-in IPQ4019 (SoC) 802.11b/g/n 2x2:2, 3 dBi internal antennae
   · Built-in IPQ4019 (SoC) 802.11a/n/ac 2x2:2, 5.5 dBi internal antennae
 - Ethernet: Built-in IPQ4019 (SoC, QCA8075) , 5x 1000/100/10 port
 - 1x USB Type A port
 - 1x Mini PCI-E port (supporting USB)
 - 1x Mini PCI-E LTE modem (MikroTik R11e-LTE6, Cat.6)

Installation:

Make sure your unit is runnning RouterOS v6 and RouterBOOT v6 (tested on 6.49.6).

0. Export your MikroTik license key (in case you want to use the device with RouterOS later)
1. Boot the initramfs image via TFTP
2. Upload the "openwrt-ipq40xx-mikrotik-mikrotik_hap-ac3-lte6-kit-squashfs-sysupgrade.bin" via SCP to the /tmp folder
3. Use sysupgrade to flash the image: sysupgrade -n /tmp/openwrt-ipq40xx-mikrotik-mikrotik_hap-ac3-lte6-kit-squashfs-sysupgrade.bin
4. Recovery to factory software is possible via Netinstall:
   https://help.mikrotik.com/docs/display/ROS/Netinstall

Signed-off-by: Csaba Sipos <metro4@freemail.hu>