ath79: add support for Longdata APS256

ath79: add support for Longdata APS256

Forum discussion : https://forum.openwrt.org/t/aps-256va-help-for-identification/143653/52

Specification:
Power: 12-36V input via 5,5/2,1 DC barrel jack, or 5V Micro USB-B
CPU: Atheros AR9344 rev 2
RAM: 128MB
Flash: 16MB
WI-Fi: 2.4GHz
Fast Ethernet: 1 WAN and 2 LAN
USB: 2 x USB-A, 1 x micro-USB-B (for power input)
WWAN: 3G modem via extended mini-PCIE form factor (can be replaced with Wifi 5GHz card)

The device come with custom openwrt BB an CC.

Because of limited LAN port, I disable GMAC0, so the WAN port can be connected to GMAC1 and function as LAN port as well.

Enable ssh access and Backup:

1.  open router admin page via LAN cable
2.  browse 192.168.111.1:8000
3.  login with password 123456
4.  click wifi icon on top menu
5.  change the path at the end of the url (after random hash) with /admin/system/flashops
        it will looks like this:
        http://192.168.111.1:8000/cgi-bin/luci/;stok=29698152cf64c980177a04f86c99ea0d/admin/system/flashops
        (the hash after "stok=" will be different)
6.  restore the config with this modified backup (can be created manually by changing dropbear config to allow ssh)
        https://drive.google.com/file/d/1Vs-k7DHBSRZFfkxv1cMOmgAPZfB-RUen/view?usp=sharing
7.  now you can login to ssh with root user and 123456 password, and backup all partition and upgrade firmware

!!! BACKUP EVERY PARTITION !!!

Flashing instructions:
- Flash directly from factory web interface accessed from "Enable ssh access" step 5

Signed-off-by: Roy H <roy@altbytes.com>
Link: https://github.com/openwrt/openwrt/pull/17939
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>