ramips: add support for TP-Link RE500 v1

ramips: add support for TP-Link RE500 v1

This device uses the same hardware as RE650 v1 which got supported in
8c51dde.

Hardware specification:

- SoC 880 MHz - MediaTek MT7621AT
- 128 MB of DDR3 RAM
- 16 MB - Winbond 25Q128FVSG
- 4T4R 2.4 GHz - MediaTek MT7615E
- 4T4R 5 GHz - MediaTek MT7615E
- 1x 1 Gbps Ethernet - MT7621AT integrated
- 7x LEDs (Power, 2G, 5G, WPS(x2), Lan(x2))
- 4x buttons (Reset, Power, WPS, LED)
- UART header (J1) - 2:GND, 3:RX, 4:TX
  Serial console @ 57600,8n1

Flash instructions:

Upload
openwrt-ramips-mt7621-tplink_re500-v1-squashfs-factory.bin
from the RE500 web interface.

TFTP recovery to stock firmware:

Unfortunately, I can't find an easy way to recover the RE
without opening the device and using modified binaries. The
TFTP upload will only work if selected from u-boot, which
means you have to open the device and attach to the serial
console. The TFTP update procedure does *not* accept the
published vendor firmware binaries. However, it allows to
flash kernel + rootfs binaries, and this works if you have
a backup of the original contents of the flash. It's probably
possible to create special image out of the vendor binaries
and use that as recovery image.

Signed-off-by: Christoph Krapp <achterin@googlemail.com>
[remove dts-v1 in DTSI, do not touch WiFi LEDs for RE650, keep
state_default in DTS files, fix label-mac-device, use lower case
for WiFi LEDs]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>