ramips: add support for HiLink HLK-7621A evaluation board

ramips: add support for HiLink HLK-7621A evaluation board

Specifications:
- SoC: MediaTek MT7621AT
- RAM: 256 MB (DDR3)
- Flash: 32 MB SPI NOR 44MHz
- Switch: 1 WAN, 4 LAN (Gigabit)
- LEDs: 1 WAN, 4 LAN (controlled by PHY)
- USB Ports: 1 x USB2, 1 x USB3
- WLAN: 1 x 2.4, 5 GHz 866Mbps (MT7612E)
- Button: 1 button (reset)
- UART Serial: UART1 as console : 57600 baud
- Power: 12VDC, 1A

Installation:
Update openWRT firmware using internal GNUBEE uboot:
https://github.com/gnubee-git/GnuBee-MT7621-uboot

By HTTP: Initial uboot address is http://10.10.10.123, your address
needs to be 10.10.10.x, and mask 255.255.255.0.
By TFTP: Uboot is in client mode, the address of the firmware must
be tftp://10.10.10.3/uboot.bin

Recovery:
Manufacturer provides MTK OpenWrt 14.07 source code, compile then
flash it by uboot.
HLK-7621A is a stamp hole package module for embedded development,
users have to design IO boards to use it.

MAC addresses:
- u-boot-env contains a placeholder address:
  > mtd_get_mac_ascii u-boot-env ethaddr
  03:17:73:ab:cd:ef
- phy0 gets a valid-looking address:
  > cat /sys/class/ieee80211/phy0/macaddress
  f8:62:aa:**:**:a8
- Calibration data for &pcie2 contains a valid address, however the
  zeros in the right half look like it's not real:
  8c:88:2b:00:00:1b
- Since it's an evaluation board and there is no solid information
  about the MAC address assignment, the ethernet MAC address is left random.

Signed-off-by: Chen Yijun <cyjason@bupt.edu.cn>
[add keys and pcie nodes to properly support evaluation board]
Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
[remove ethernet address, wrap lines properly]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>