ipq40xx: add MikroTik wAP ac (RBwAPG-5HacD2HnD) support

ipq40xx: add MikroTik wAP ac (RBwAPG-5HacD2HnD) support

The MikroTik wAP ac (RBwAPG-5HacD2HnD) is a dual-band dual-radio
802.11ac wireless access point with integrated antenna and two Ethernet
ports in a weatherproof enclosure. See
https://mikrotik.com/product/wap_ac for more information.

Important: this is the new ipq40xx-based wAP ac, not the older
ath79-based wAP ac (RBwAPG-5HacT2HnD), already supported in OpenWrt.

Specifications:
 - SoC: Qualcomm Atheros IPQ4018
 - CPU: 4x ARM Cortex A7
 - RAM: 128MB
 - Storage: 16MB NOR flash
 - Wireless
    - 2.4GHz: Built-in IPQ4018 (SoC) 802.11b/g/n 2x2:2, 2.5 dBi antennae
    - 5GHz: Built-in IPQ4018 (SoC) 802.11a/n/ac 2x2:2, 2.5 dBi antennae
 - Ethernet: Built-in IPQ4018 (SoC, QCA8075), 2x 1000/100/10Mb/s ports,
   one with 802.3af/at PoE in

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

Notes:
This preserves the MAC addresses of the physical Ethernet ports:
 - eth0 corresponds to the physical port labeled ETH1 and has the base
   MAC address. This port can be used to power the device.
 - eth1 corresponds to the physical port labeled ETH2 and has a MAC
   address one greater than the base.

MAC addresses are set from /lib/preinit/05_set_iface_mac_ipq40xx.sh
rather than /etc/board.d/02_network so that they are in effect for
preinit. This should likely be done for other MikroTik devices and
possibly other non-MikroTik devices as well.

As this device has 2 physical ports, they are each connected to their
respective PHYs, allowing the link status to be visible to software.
Since they are not marked on the case with any role (such as LAN or
WAN), both are bridged to the lan network by default, although this can
easily be changed if needed.

Signed-off-by: Mark Mentovai <mark@mentovai.com>