ramips: add support for ALFA Network Tube-E4G

ramips: add support for ALFA Network Tube-E4G

ALFA Network Tube-E4G is an outdoor, dual-SIM LTE Cat. 4 CPE, based on
MediaTek MT7620A, equipped with Quectel EC25 miniPCIe modem.

Specification:

- MT7620A (580 MHz)
- 64/128/256 MB of RAM (DDR2)
- 16/32 MB of flash (SPI NOR)
- 1x 10/100 Mbps Ethernet, with passive PoE support (24 V)
- 1x miniPCIe slot (with PCIe and USB 2.0 buses)
- 2x SIM slot (mini, micro) with detect and switch driven by GPIO
- 1x detachable antenna (modem main)
- 1x internal antenna (modem div)
- 1x GPS passive antenna (optional)
- 5x LED (all driven by GPIO)
- 1x button (reset)
- UART (4-pin, 2.54 mm pitch) header on PCB

Other:

Default SIM slot is selected at an early stage by U-Boot, based on
'default_sim' environment value: 1 or unset = SIM1 (mini), 2 = SIM2
(micro). U-Boot also resets the modem, using #PERST signal, before
starting kernel.

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 LAN 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>