realtek: add support for EnGenius EWS2910P

realtek: add support for EnGenius EWS2910P

Add support for the Engenius EWS2910P PoE switch. This is an RTL8380
based switch with two SFP slots, and PoE 802.3af one every RJ-45 port.

The specs say 802.3af, but the vendor firmware configures the PSE for
a budget of 31W, indicating 802.3at support.

Specifications:
---------------
* SoC:       Realtek RTL8380M
* Flash:     32 MiB SPI flash Macronix MX25L25635E
* RAM:       256 MiB (As reported by bootloader)
* Ethernet:  16x 10/100/1000 Mbps with PoE
              2x SFP slots
* Buttons:   1 "Reset" button on front panel
             1 "LED mode: button on front panel
             1 "On/Off" Toggle switch on the back
* Power:     48V-54V DC barrel jack
* UART:      1 serial header (JP1) with populated 2.54mm pitch header
             Labeled GRTV for ground, rx, tx, and 3.3V respectively
* PoE:       1 STM ST32F100 microcontroller
             2 BCM59111 PSE chips
Works:
------
  - (8) RJ-45 ethernet ports
  - Switch functions
  - LEDs and buttons

Not yet enabled:
----------------
  - SFP ports (will be enabled in a subsequent change)
  - Power-over-Ethernet (requires realtek-poe package)

Install via web interface:
-------------------------

The factory firmware will accept and flash the initramfs image. It is
recommended to flash to "Partition 0". Flashing to "Partition 1" is
not supported at this point.

The factory web GUI will show the following warning:

 " Warning: The firmware version is v0.00.00-c0.0.00
     The firmware image you are uploading is older than the current
     firmware of the switch. The device will reset back to default
     settings. Are you sure you want to proceed?"

This is expected when flashing OpenWrt. After the initramfs image
boots, flash the -sysupgrade using either the commandline or LuCI.

Install via serial console/tftp:
--------------------------------

The u-boot firmware will not stop the boot, regardless of which key is
pressed. To access the u-boot console, ground out the CLK (pin 16) of
the ROM (U22) when u-boot is reading the linux image. If timed
correctly, the image CRC will fail, and u-boot will drop to a shell:
    > rtk network on
    > setenv ipaddr <address of tftp server>
    > tftp $(freemem) <name-of-initramfs-image.bin>
    > bootm

Then flash the -sysupgrade using either the commandline or luci.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
[gpio-led node names, OpenWrt and LuCI capitalization in commit message]
Signed-off-by: Sander Vanheule <sander@svanheule.net>