ramips: add support for Youku X2

ramips: add support for Youku X2

Specifications:
  SOC:      MT7620AN + MT7612EN
  RAM:      128 MiB DDR2
  Flash:    16 MiB (Winbond W25Q28FVFG)
  WLAN:     2.4G + 5G
  LAN:      LAN ports *2
  WAN:      WAN port *1
  USB:      USB2.0 *1
  SD Card:  MicroSD *1
  Buttons:  Reset *1
  LEDs: ethernet *3, system, usb, wlan2g, wlan5g

MAC Address:
  use        address               source
  label      54:36:9b:xx:xx:ac     lan
  lan        54:36:9b:xx:xx:ac     factory.0x0028
  wan        54:36:9b:xx:xx:ad     factory.0x002e
  wlan2g     54:36:9b:xx:xx:ae     factory.0x0004
  wlan5g     54:36:9b:xx:xx:af     factory.0x8004

Installation:
1. Apply initramfs-kernel.bin in stock firmware Web UI.
2. Install sysupgrade.bin on OpenWrt and do not retain any configuration.

Signed-off-by: Shiji Yang <yangshiji66@qq.com>