ramips: add support for Linksys E5400 and clones

ramips: add support for Linksys E5400 and clones

Linksys E5400 is a 2.4/5GHz dual band AC router, based on MediaTek
MT7628AN. This device is also sold as Linksys E2500v4, E5300 and E5350
with the same hardware, but software speed limitations in vendor
firmwares.

Specification:
 * SoC: MT7628AN (580 MHz)
 * RAM: DDR2 64 MiB
 * Flash: 16 MiB NOR (W25Q128BV)
 * Wi-Fi:
   * 2.4GHz: SoC Built-in
   * 5GHz: MT7612EN
 * Ethernet: 5x 100Mbps
   * Switch: SoC built-in
 * UART:
   * 115200, 8N1, 3.3V (real u-boot speed: 119050)
   * Pinout JB4: 1:[3V3] (TXD) (RXD) [NC] (GND)

Flash Layout:
  0x0000000-0x0030000 : "Bootloader"
  0x0030000-0x0040000 : "Config"
  0x0040000-0x0050000 : "Factory"
  0x0050000-0x0ff0000 : "Kernel"
  0x0ff0000-0x1000000 : "CBTinfo"

MAC address:
  LAN: factory 0x28
  WAN: LAN + 1
  2.4G: LAN + 2
  5G: LAN + 3

Installation:
1. Connect to UART, use LF as EOL (not CRLF)
2. Set IP address
  - Press 4 during boot
  - setenv serverip <TFTPSERVER_IP>
  - setenv ipaddr <DEVICE_IP>
  - setenv bootfile openwrt-ramips-mt76x8-linksys_e5400-initramfs-kernel.bin
  - saveenv
  - reset
3. Load Openwrt Kernel image from TFTP:
  - Press 1 during boot
  - IP and filename should be already set
4. Boot into OpenWrt and perform sysupgrade with sysupgrade image.

Signed-off-by: Julien Dusser <julien.dusser@free.fr>