ar71xx: complete support for RB wAP 2nD

ar71xx: complete support for RB wAP 2nD

This patch adds support for the MikroTik RouterBOARD wAP
https://mikrotik.com/product/RBwAP2nD

Specifications:

- SoC: Qualcomm QCA9533 (650 MHz)
- RAM: 64 MB
- Storage: 16 MB NOR SPI flash
- Wireless: built-in QCA9533, 2x2:2
- Ethernet: 1x100M (802.3af/at POE in)

This patch adds missing code to fully support wAP. Machfile already
contained configuration for wAP 2nD but device specific configuration
like LEDs etc. was missing.

Installation:

1. Login to the Mikrotik WebUI to backup your licence keys
2. Setup a DHCP/BOOTP server with:
   - DHCP-Option 66 (TFTP server name) pointing to a local TFTP
     server within the same subnet of the DHCP range
   - DHCP-Option 67 (Bootfile-Name) matching the initramfs filename
     of the to be booted image
3. Connect the port labeled internet to your local network
4. Keep the reset button pushed down and power on the board

The board should load and start the initramfs image from the TFTP
server. Login as root/without password to the started LEDE via SSH
listing on IPv4 address 192.168.1.1. Use sysupgrade to install LEDE.

Revert to RouterOS

Use the "rbcfg" package on in LEDE:
- rbcfg set boot_protocol bootp
- rbcfg set boot_device ethnand
- rbcfg apply

Open Netinstall and reboot routerboard. Now Netinstall sees RouterBOARD
and you can install RouterOS. If NetInstall gets stuck on Sending offer
just wait for it to timeout and then close and open Netinstall again.

Click on install again.

In order for RouterOS to function properly, you need to restore license
for the device. You can do that by including license in NetInstall.

Signed-off-by: Robert Marko <robimarko@gmail.com>