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>

target/linux/ar71xx/base-files/lib/ar71xx.sh

@@ -526,6 +526,9 @@ ar71xx_board_detect() {
 		name="cpe510"
 		tplink_pharos_board_detect
 		;;
+	*CPE870)
+		name="cpe870"
+		;;
 	*CR3000)
 		name="cr3000"
 		;;

target/linux/ar71xx/base-files/lib/upgrade/platform.sh

@@ -188,6 +188,7 @@ platform_check_image() {
 	cf-e380ac-v2|\
 	cf-e520n|\
 	cf-e530n|\
+	cpe870|\
 	dgl-5500-a1|\
 	dhp-1565-a1|\
 	dir-505-a1|\