ath79: add support for D-Link DAP-2695-A1

ath79: add support for D-Link DAP-2695-A1

Hardware:
* SoC: Qualcomm Atheros QCA9558
* RAM: 256MB
* Flash: 16MB SPI NOR
* Ethernet: 2x 10/100/1000 (1x 802.3at PoE-PD)
* WiFi 2.4GHz: Qualcomm Atheros QCA9558
* WiFi 5GHz: Qualcomm Ahteros QCA9880-2R4E
* LEDS: 1x 5GHz, 1x 2.4GHz, 1x LAN1(POE), 1x LAN2, 1x POWER
* Buttons: 1x RESET
* UART: 1x RJ45 RS-232 Console port

Installation via stock firmware:
* Install the factory image via the stock firmware web interface

Installation via bootloader Emergency Web Server:
* Connect your PC to the LAN1(PoE) port
* Configure your PC with IP address 192.168.0.90
* Open a serial console to the Console port (115200,8n1)
* Press "q" within 2s when "press 'q' to stop autoboot" appears
* Open http://192.168.0.50 in a browser
* Upload either the factory or the sysupgrade image
* Once you see "write image into flash...OK,dest addr=0x9f070000" you
  can power-cycle the device. Ignore "checksum bad" messages.

Setting the MAC addresses for the ethernet interfaces via
/etc/board.d/02_network adds the following snippets to
/etc/config/network:

config device 'lan_eth0_1_dev'
        option name 'eth0.1'
        option macaddr 'xx:xx:xx:xx:xx:xx'

config device 'wan_eth1_2_dev'
        option name 'eth1.2'
        option macaddr 'xx:xx:xx:xx:xx:xx'

This would result in the proper MAC addresses being set for the VLAN
subinterfaces, but the parent interfaces would still have a random MAC
address. Using untagged VLANs could solve this, but would still leave
those extra snippets in /etc/config/network, and then the device VLAN
setup would differ from the one used in ar71xx. Therefore, the MAC
addresses of the ethernet interfaces are being set via preinit instead.

The bdcfg partition contains 4 MAC address labels:
 - lanmac
 - wanmac
 - wlanmac
 - wlanmac_a

The first 3 all contain the same MAC address, which is also the one on
the label.

Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Reviewed-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>