ath79: add support for Yuncore A930

ath79: add support for Yuncore A930

Specification:

- QCA9533 (650 MHz), 64 or 128MB RAM, 16MB SPI NOR
- 2x 10/100 Mbps Ethernet, with 802.3at PoE support (WAN)
- 2T2R 802.11b/g/n 2.4GHz

Flash instructions:

If your device comes with generic QSDK based firmware, you can login
over telnet (login: root, empty password, default IP: 192.168.188.253),
issue first (important!) 'fw_setenv' command and then perform regular
upgrade, using 'sysupgrade -n -F ...' (you can use 'wget' to download
image to the device, SSH server is not available):

  fw_setenv bootcmd "bootm 0x9f050000 || bootm 0x9fe80000"
  sysupgrade -n -F openwrt-...-yuncore_...-squashfs-sysupgrade.bin

In case your device runs firmware with YunCore custom GUI, you can use
U-Boot recovery mode:

1. Set a static IP 192.168.0.141/24 on PC and start TFTP server with
   'tftp' image renamed to 'upgrade.bin'
2. Power the device with reset button pressed and release it after 5-7
   seconds, recovery mode should start downloading image from server
   (unfortunately, there is no visible indication that recovery got
   enabled - in case of problems check TFTP server logs)

Signed-off-by: Clemens Hopfer <openwrt@wireloss.net>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
(cherry-picked from commit a05dcb07241aa83a4416b56201e31b4af8518981)
[switch to mtd-mac-address instead of nvmem-cells]