ath79: add support for Letv LBA-047-CH

ath79: add support for Letv LBA-047-CH

Specifications:
SOC: QCA9531 650 MHz
ROM: 16 MiB Flash (Winbond W25Q128FV)
RAM: 128 MiB DDR2 (Winbond W971GG6SB)
LAN: 10/100M *2
WAN: 10/100M *1
LED: BGR color *1

Mac address:
label	C8:0E:77:xx:xx:68	art@0x0
lan	C8:0E:77:xx:xx:62	art@0x6
wan	C8:0E:77:xx:xx:68	art@0x0    (same as the label)
wlan	C8:0E:77:xx:xx:B2	art@0x1002 (load automatically)

TFTP installation:
* Set local IP to 192.168.67.100 and open tftpd64, link lan
  port to computer.
  Rename "xxxx-factory.bin" to
  "openwrt-ar71xx-generic-ap147-16M-rootfs-squashfs.bin".
* Make sure firmware file is in the tftpd's directory, push
  reset button and plug in, hold it for 5 seconds, and then
  it will download firmware from tftp server automatically.

More information:
* This device boot from flash@0xe80000 so we need a okli
  loader to deal with small kernel partition issue. In order
  to make full use of the storage space, connect a part of the
  previous kernel partition to the firmware.

  Stock                          Modify
  0x000000-0x040000(u-boot)      0x000000-0x040000(u-boot)
  0x040000-0x050000(u-boot-env)  0x000000-0x050000(u-boot-env)
  0x050000-0xe80000(rootfs)      0x050000-0xe80000(firmware part1)
  0xe80000-0xff0000(kernel)      0xe80000-0xe90000(okli-loader)
                                 0xe90000-0xff0000(firmware part2)
  0xff0000-0x1000000(art)        0xff0000-0x1000000(art)

Signed-off-by: Shiji Yang <yangshiji66@qq.com>