ramips: add support for Edimax Gemini RE23S

ramips: add support for Edimax Gemini RE23S

Hardware
--------
SoC:   Mediatek MT7621AT (880 MHz, 2 cores 4 threads)
RAM:   128MB
FLASH: 16MB NOR (Macronix MX25L12805D)
ETH:   1x 10/100/1000 Mbps Ethernet (MT7530)
WIFI:
  - 2.4GHz: 1x MT7615 (4x4:4)
  - 5GHz:   1x MT7615 (4x4:4)
  - 4 antennas: 2 external detachable and 2 internal
BTN:
  - 1x Reset button
  - 1x WPS button
LEDS:
  - 1x Green led (Power)
  - 1x Green-Amber-Red led (Wifi)
UART:
  - 57600-8-N-1

Everything works correctly.

Installation
------------
Flash the factory image directly from OEM web interface.
(You can login using these credentials: admin/1234)

Restore OEM Firmware
--------------------
Flash the OEM "bin" firmware directly from LUCI.
The firmware is downloadable from the OEM web page.
Warning: Remember to not keep settings!
Warning2: Remember to force the flash.

Restoring procedure tested with RE23_1.08.bin

MAC addresses
-------------
factory 0x4     *:24
factory 0x8004  *:25
Cimage  0x07    *:24
Cimage  0x0D    *:24
Cimage  0x13    *:24
Cimage  0x19    *:25

No other addresses were found in factory partition.

Since the label contains both the 2.4GHz and 5GHz mac address I decided
to set the 5GHz one as label-mac-device. Moreover it also corresponds
to the lan mac address.

Notes
-----
The wifi led in the OEM firmware changes colour depending on the signal
strength. This can be done in OpenWrt but just for one interface.
So for now will not be any default action for this led.

If you want to open the case, pay attention to the antenna placed on
the bottom part of the front cover.
The wire is a bit short and it breaks easily. (I broke it)

Signed-off-by: Davide Fioravanti <pantanastyle@gmail.com>
[fix two typos and add extended MAC address section to commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>