ath79: add support for TP-Link Archer C2 V3 (AC900)

ath79: add support for TP-Link Archer C2 V3 (AC900)

This commit adds support for the TP-link Archer C2 V3 (AC900).

Specifications:

 - CPU: QCA9563 750Mhz
 - Ram: 64MB (DDR2)
 - Flash: 8MB (SPI NOR)
 - Ethernet: 5x 10/100/1000
 - Wifi: QCA9563 bgn + QCA9887 an+ac
 - 9x Leds, 2x buttons

Flash instructions:

Upload openwrt-ath79-generic-tplink_archer-c2-v3-squashfs-factory.bin
via the router Web interface and flash as normal firmware update.

Signed-off-by: Skirmantas Lauzikas <skirmantas.lauzikas@blackraven.lt>
[reorder entries, fix trailing whitespace in dts, add firmware partition
 compatible string, remove led default off states, use default on state
 for system led, reuse tplink-safeloader-uimage image build recipe, squash
 with dynamic partitioning commit, reword commit message, add
 installation instructions]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>