ath79: add support for TP-Link Archer C6 v2 (US) and A6 (US/TW)

ath79: add support for TP-Link Archer C6 v2 (US) and A6 (US/TW)

This patch is based on #1689 and adds support for TP-Link Archer
C6 v2 (US) and A6 (US/TW).

The hardware is the same as EU and RU variant, except for GPIOs
(LEDS/Buttons), flash(chip/partitions) and UART being available
on the board.

- SOC: Qualcomm QCA9563 @ 775MHz
- Flash: GigaDevice GD25Q127CS1G (16MiB)
- RAM: Zentel A3R1GE40JBF (128 MiB DDR2)
- Ethernet: Qualcomm QCA8337N: 4x 1Gbps LAN + 1x 1Gbps WAN
- Wireless:
  - 2.4GHz (bgn) QCA9563 integrated (3x3)
  - 5GHz (ac) Qualcomm QCA9886 (2x2)
- Button: 1x power, 1x reset, 1x wps
- LED: 6x LEDs: power, wlan2g, wlan5g, lan, wan, wps
- UART: 115200, 8n1 (header available on board)

Known issues:
 - Wireless: 5GHz is known to have lower RSSI signal, it affects speed and range.

Flash instructions:

Upload openwrt-ath79-generic-tplink_archer-c6-v2-us-squashfs-factory.bin
via the router Web interface.

Flash instruction using tftp recovery:

1. Connect the computer to one of the LAN ports of the router
2. Set the computer IP to 192.168.0.66
3. Start a tftp server with the OpenWrt factory image in the
   tftp root directory renamed to ArcherA6v2_tp_recovery.bin.
4. Connect power cable to router, press and hold the
   reset button and turn the router on
5. Keep the reset button pressed until the WPS LED lights up
6. Wait ~150 seconds to complete flashing

Flash partitioning: I've followed #1689 for defining the partition layout
for this patch. The partition named as "tplink" @ 0xfd0000 is marked
as read only as it is where some config for stock firmware are stored.
On stock firmware those stock partitions starts at 0xfd9400 however
I had not been able to make it functional starting on the same address as
on stock fw, so it has been partitioned following #1689 and not the stock
partition layout for this specific partition. Due to that firmware/rootfs
partition lenght is 0xf80000 and not 0xf89400 as stock.

According to the GPL code, the EU/RU/JP variant does have different GPIO pins
assignment to LEDs and buttons, also the flash memory layout is different.

GPL Source Code: https://static.tp-link.com/resources/gpl/gpl-A6v2_us.tar.gz

Signed-off-by: Anderson Vulczak <andi@andi.com.br>
[wrap commit message, remove soft_ver change for C6 v2 EU, move LED aliases
to DTS files, remove dts-v1 in DTSI, node/property reorder in DTSI]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>