ramips: Add support for the GnuBee Personal Cloud Two

ramips: Add support for the GnuBee Personal Cloud Two

The GnuBee Personal Cloud Two crowdfunded on https://www.crowdsupply.com
It is a low-cost, low-power, network-attached storage device.

Specifications:

- SoC: MediaTek MT7621AT
- RAM: DDR3 512 MB
- Flash: 32 MB
- Six SATA ports for 3.5" Drives
- One SDcard
- One USB 3.0
- Two USB 2.0
- Gigabit Ethernet: Three Ports
- UART 3.5mm Audio Jack or 3 pin header - 57600 8N1
- Three GPIOs available on a pin header

Flash instructions:

The GnuBee Personal Cloud Two ships with libreCMC installed.
libreCMC is a Free Software Foundation approved fork of LEDE/OpenWrt.
As such one can upgrade using the webinterface or sysupgrade.

Das U-Boot has multiple options for recovery or updates including :

- USB
- http
- tftp

Errata:
- While there are three ethernet ports, the third requires support for
  the second GMAC. This will come in kernel 4.14.
- The first hard drive slot has a clearance issue with the two fan
  headers. Workaround is to pull the headers out and connect the pins to
  jumper wires.
- Using this device as a NAS is problematic with the 4.9 kernel as many
  /dev/sdX reads throw silent errors. The current theory behind this is
  some kind of unhandled DMA mapping error in the kernel. This is not an
  issue with kernel 4.4.

Signed-off-by: L. D. Pinney <ldpinney@gmail.com>
Signed-off-by: Rosen Penev <rosenp@gmail.com>