ar71xx: add support for YunCore CPE870

YunCore CPE870 is an outdoor CPE/AP based on Atheros AR9341.
Short specification:

- 535/400/200 MHz (CPU/DDR/AHB)
- 2x 10/100 Mbps Ethernet, passive PoE support
- 64/128 MB of RAM (DDR2)
- 8 MB of FLASH
- 2T2R 2.4 GHz with external PA (SKY65174-21), up to 30 dBm
- internal 14 dBi panel antenna
- 8x LED, 1x button
- UART (JP1) header on PCB

Flash instruction (do it under U-Boot, using UART):

1. tftp 0x80060000 lede-ar71xx-generic-cpe870-squashfs-sysupgrade.bin
2. erase 0x9f020000 +$filesize
3. cp.b $fileaddr 0x9f020000 $filesize
4. setenv bootcmd "bootm 0x9f020000"
5. saveenv && reset

Vendor firmware access (login/password): Admin/5up

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>