ramips: add support for Linksys RE7000

ramips: add support for Linksys RE7000

Hardware specification:

- SoC: MediaTek MT7621AT (880 MHz)
- Flash: 16 MB (Macronix MX25L12835FM2I-10G)
- RAM: 128 MB (Nanya NT5CC64M16GP-DI)
- WLAN 2.4 GHz: 2x2 MediaTek MT7603EN
- WLAN 5 GHz: 2x2 MediaTek MT7615N
- Ethernet: 1x 10/100/1000 Mbps
- LED: Power, Wifi, WPS
- Button: Reset, WPS
- UART: 1:VCC, 2:GND, 3:TX, 4:RX (from LAN port)
  Serial console @ 57600,8n1

Flash instructions:

Connect to serial console and start up the device. As the bootloader got
locked you need to type in a password to unlock U-Boot access.
When you see the following output on the console:

relocate_code Pointer at: 87f1c000

type in the super secure password:

1234567890

Then select TFTP boot from RAM by selecting option 1 in the boot menu.
As Linksys decided to leave out a basic TFTP configuration you need to
set server- & client ip as well as the image filename the device will
search for. You need to use the initramfs openwrt image for the TFTP
boot process.

Once openwrt has booted up, upload the sysupgrade image via scp and run
sysupgrade as normal.

Signed-off-by: Christoph Krapp <achterin@gmail.com>