ramips: Add support for Head Weblink HDRM200

ramips: Add support for Head Weblink HDRM200

Head Weblink HDRM200 is a dual-sim router based on MT7620A. The detailed
specifications are:

- MT7620A (580MHz)
- 64MB RAM
- 16MB of flash (SPI NOR)
- 6x 10/100Mbps Ethernet (MT7620A built-in switch)
- 1x microSD slot
- 1x miniPCIe slot (only USB2.0 bus). Device is shipped with a SIMCOM
SIM7100E LTE modem.
- 2x SIM slots (standard size)
- 1x USB2.0 port
- 1x 2.4GHz wifi (rt2800)
- 1x 5GHz wifi (mt7612)
- 1x reset button
- 1x WPS button
- 3x GPIO-controllable LEDs
- 1x 10 pin terminal block (RS232, RS485, 4 x GPIO)

Tested:
- Ethernet switch
- Wifi
- USB slot
- SD card slot
- miniPCIe-slot
- sysupgrade
- reset button

Installation instructions:

Installing OpenWRT for the first time requires a bit of work, as the
board does not ship with OpenWRT. In addition, the bootloader
automatically reboots when installing an image over tftp. In order to
install OpenWRT on the HDRM200, you need to do the following:

* Copy the initramfs-image to your tftp-root (default filename is
test.bin) and configure networking accordingly (default server IP is
10.10.10.3, client 10.10.10.123). Start your tftp server.
* Open the board and connect to UART. The pins are exposed and clearly
marked.
* Boot the board and press 1.
* Either use the default filename and client/server IP-addresses, or
specify your own.

The image should now be loaded to memory and board boot. If the router
reboots while the image is loading, you need to try again. Once the
board has booted, copy the sysupgrade-image to the router and run
sysupgrade in order to install OpenWRT to the flash.

Notes:

- You control which SIM slot to use by writing 0/1 to
/sys/class/gpio/gpio0/value. In order for the change to take
effect, you can either use AT-commands (AT+CFUN) or power-cycle the
modem (write 0/1 to /sys/class/gpio/gpio21/value).
- RS485 is available on /dev/ttyS0.
- RS232 is available on /dev/ttyS1.
- The name of the ioX-gpios map to the labels on the casing.

Signed-off-by: Kristian Evensen <kristian.evensen@gmail.com>
[fixed whitespace issue and merge conflict in target.mk]
Signed-off-by: Petr Štetiar <ynezz@true.cz>