ramips: add support for ALFA Network R36M-E4G

ramips: add support for ALFA Network R36M-E4G

ALFA Network R36M-E4G is a dual-SIM, N300 Wi-Fi, compact size platform
based on MediaTek MT7620A WiSoC. This product is designed for operation
with 4G modem (can be bought in bundle with Quectel EC25, EG25 or EP06)
but supports also Wi-Fi modules (miniPCIe slot has USB and PCIe buses).

Specification:

- MT7620A (580 MHz)
- 64/128/256 MB of RAM (DDR2)
- 16/32+ MB of FLASH (SPI NOR)
- 2x 10/100 Mbps Ethernet, with passive PoE support (24 V)
- 2T2R 2.4 GHz (MT7620A), with ext. LNA (RFFM4227)
- 1x miniPCIe slot (with PCIe and USB 2.0 buses and optional 5 V)
- 2x SIM slot (mini, micro) with detect and switch driven by GPIO
- 2x u.fl antenna connectors (for Wi-Fi)
- 8x LED (7 driven by GPIO)
- 2x button (reset, wifi)
- 2x UART (4-pin/2.54 mm pitch, 10-pin/1.27 mm pitch) headers on PCB
- 1x I2C (4-pin, 1.27 mm pitch) header on PCB
- 1x LED (8-pin, 1.27 mm pitch) header on PCB
- 1x DC jack with lock (12 V)

Other:

- there is a dedicated, 4-pin connector for optional RTC module (Holtek
  HT138x) with 'enable' input, not available at the time of preparing
  support for this board
- miniPCIe slot supports additional 5 V supply on pins 47 and 49 but a
  jumper resistor (R174) is not installed by default
- U-Boot selects default SIM slot, based on value of 'default_sim' env
  variable: '1' or unset -> SIM1 (mini), '2' -> SIM2 (micro). This will
  work only if both slots are occupied, otherwise U-Boot will always
  select slot with SIM card inside (user can override it later, in
  user-space)
- U-Boot resets the modem, using PERSTn signal, before starting kernel
- this board supports 'dual image' feature (controlled by 'dual_image'
  U-Boot environment variable)

Flash instruction:

You can use the 'sysupgrade' image directly in vendor firmware which is
based on OpenWrt (make sure to not preserve settings - use 'sysupgrade
-n -F ...' command). Alternatively, use web recovery mode in U-Boot:

1. Power the device with reset button pressed, the modem LED will start
   blinking slowly and after ~3 seconds, when it starts blinking faster,
   you can release the button.
2. Setup static IP 192.168.1.2/24 on your PC.
3. Go to 192.168.1.1 in browser and upload 'sysupgrade' image.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>