ath79: add support for D-Link DAP-3662 A1

ath79: add support for D-Link DAP-3662 A1

Specifications:
 * QCA9557, 16 MiB Flash, 128 MiB RAM, 802.11n 2T2R
 * QCA9882, 802.11ac 2T2R
 * 2x Gigabit LAN (1x 802.11af PoE)
 * IP68 pole-mountable outdoor case

Installation:
 * Factory Web UI is at 192.168.0.50
   login with 'admin' and blank password, flash factory.bin
 * Recovery Web UI is at 192.168.0.50
   connect network cable, hold reset button during power-on and keep it
   pressed until uploading has started (only required when checksum is ok,
   e.g. for reverting back to oem firmware), flash factory.bin

After flashing factory.bin, additional free space can be reclaimed by
flashing sysupgrade.bin, since the factory image requires some padding
to be accepted for upgrading via OEM Web UI.

Both ethernet ports are set to LAN by default, matching the labelling on
the case. However, since both GMAC Interfaces eth0 and eth1 are connected
to the switch (QCA8337), the user may create an additional 'wan' interface
as desired and override the vlan id settings to map br-lan / wan to either
the PoE or non-PoE port, depending on the individual scenario of use.

So, the LAN and WAN ports would then be connected to different GMACs, e.g.

config interface 'lan'
	option ifname 'eth0.1'
	...

config interface 'wan'
	option ifname 'eth1.2'
	...

config switch_vlan
        option device 'switch0'
        option vlan '1'
        option ports '1 0t'

config switch_vlan
        option device 'switch0'
        option vlan '2'
        option ports '2 6t'

Signed-off-by: Sebastian Schaper <openwrt@sebastianschaper.net>
[add configuration example]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>