ramips: add support for Ubiquiti EdgeRouter X-SFP

ramips: add support for Ubiquiti EdgeRouter X-SFP

This patch adds support for the Ubiquiti EdgeRouter X-SFP and
improves support for the EdgeRouter X (PoE-passthrough).

Specification:
- SoC: MediaTek MT7621AT
- Flash: 256 MiB
- RAM: 265 MiB
- Ethernet: 5 x LAN (1000 Mbps)
- UART: 1 x UART on PCB (3.3V, RX, TX, GND) - 57600 8N1
- EdgeRouter X:
  - 1 x PoE-Passtrough (Eth4)
  - powered by Wallwart or passive PoE
- EdgeRouter X-SFP:
  - 5 x PoE-Out (24V, passive)
  - 1 x SFP (unknown status)
  - powered by Wallwart (24V)

Doesn't work:
* SoC has crypto engine but no open driver.
* SoC has nat acceleration, but no open driver.
* This router has 2MB spi flash soldered in but MT
  nand/spi drivers do not support pin sharing,
  so it is not accessable and disabled. Stock
  firmware could read it and it was empty.

Installation

via vendor firmware:
- build an Initrd-image (> 3MiB) and upload the factory-image
- initrd can have luci-mod-failsafe
- flash final firmware via LuCI / sysupgrade on rebooted system

via TFTP:
- stop uboot into tftp-load into option "1"
- upload factory.bin image

Signed-off-by: Sven Roederer <devel-sven@geroedel.de>