ramips: add support for ALFA Network Quad-E4G

ramips: add support for ALFA Network Quad-E4G

ALFA Network Quad-E4G is a universal Wi-Fi/4G platform, which offers
three miniPCIe (PCIe, USB 2.0, SIM) and a single M.2 B-key (dual-SIM,
USB 3.0) slots, RTC and five Gigabit Ethernet ports with PoE support.

Specification:

- MT7621A (880 MHz)
- 256/512 MB of RAM (DDR3)
- 16/32+ MB of FLASH (SPI NOR)
- optional second SPI flash (8-pin WSON/SOIC)
- 1x microSD (SDXC) flash card reader
- 5x 10/100/1000 Mbps Ethernet, with passive PoE support (24 V) in LAN1
- optional 802.3at/af PoE module for WAN
- 3x miniPCIe slot (with PCIe and USB 2.0 buses, micro SIM and 5 V)
- 1x M.2/NGFF B-key 3042 (USB 3.0/2.0, mini + micro SIM)
- RTC (TI BQ32002, I2C bus) with backup battery (CR2032)
- external hardware watchdog (EM Microelectronic EM6324)
- 1x USB 2.0 Type-A
- 1x micro USB Type-B for system serial console (Holtek HT42B534)
- 11x LED (5 for Ethernet, 5 driven by GPIO, 1x power indicator)
- 3x button (reset, user1, user2)
- 1x I2C (4-pin, 2.54 mm pitch) header on PCB
- 4x SIM (6-pin, 2.00 mm pitch) headers on PCB
- 2x UART2/3 (4-pin, 2.54 mm pitch) headers on PCB
- 1x mechanical power switch
- 1x DC jack with lock (24 V)

Other:

- U-Boot selects default SIM slot, based on value of 'default_sim' env
  variable: '1' or unset -> SIM1 (mini), '2' -> SIM2 (micro). This board
  has additional logic circuit for M.2 SIM switching. The 'sim-select'
  will work only if both SIM slots are occupied. Otherwise, always slot
  with SIM inside is selected, no matter 'sim-select' value.
- U-Boot enables power in all three miniPCIe and M.2 slots before
  loading the kernel
- this board supports 'dual image' feature (controlled by 'dual_image'
  U-Boot environment variable)
- all three miniPCIe slots have additional 5 V supply on pins 47 and 49
- the board allows to install up to two oversized miniPCIe cards (vendor
  has dedicated MediaTek MT7615N/D cards for this board)
- this board has additional logic circuit controlling PERSTn pins inside
  miniPCIe slots. By default, PERSTn (GPIO19) is routed to all miniPCIe
  slots but setting GPIO22 to high allows PERSTn control per slot, using
  GPIO23-25 (value is inverted)

Flash instructions:

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>
(backported from commit e68539aca43a560077364de27eb220f1d85ca3ef)