ramips: add support for Vonets VAR11N-300

ramips: add support for Vonets VAR11N-300

The VAR11N-300 is a tiny wireless-N device with a hardwired Ethernet
cable, one extra Ethernet port, and an internal antenna, based on the
MediaTek MT7620n chipset.

Specs:
- MT7620n WiSoC @ 600MHz
- 32 MB SDRAM
- 4 MB SPI flash
- 2T2R 2.4GHz WiFi-N
- 1 attached 10/100 Ethernet cable (LAN)
- 1 10/100 Ethernet port (WAN)
- 1 attached USB / barrel 5vdc power cable
- 5 LEDs (see notes below)
- 1 reset button
- 1 UART (3 pads on board)

Installation:

The stock firmware does not support uploading new firmware directly,
only checking the manufacturer's site for updates.  This process may be
possible to spoof, but the update check uses some kind of homebrew
encryption that I didn't investigate.  Instead, you can install via a
backdoor:

1. Set up a TFTP server to serve the firmware binary
(lede-ramips-mt7620-var11n-300-squashfs-sysupgrade.bin)
2. Factory reset the device by holding the reset button for a few
seconds.
3. Open the web interface (default IP: 192.168.253.254)
4. Log in with the "super admin" credentials: username `vonets`,
password `vonets26642519`.
5. On the "Operative Status" page, click the text "System Uptime", then
quickly click the uptime value.
6. If successful, an alert dialog will appear reading "Ated start", and
the device will now accept telnet connections.  If the alert does not
appear, repeat step 5 until it works (the timing is a bit tricky).
7. Telnet to the device using credentials "admin / admin"
8. Retrieve the firmware binary from the tftp server: `tftp -l lede.bin
-r lede-ramips-mt7620-var11n-300-squashfs-sysupgrade.bin -g
<tftp-server-ip>`
9. Write the firmware to flash: `mtd_write write lede.bin /dev/mtd4`
10. Reboot

Tested:
- LAN / WAN ethernet
- WiFi
- LAN / WAN / status LED GPIOs (see notes below)
- Reset button
- Sysupgrade

Notes:

LEDs:

The board has 5 LEDs - two green LEDs for LAN / WAN activity, one blue
LED for WiFi, and a pair of "status" LEDs connected to the same GPIO
(the blue LED lights when the GPIO is low, and the green when it's
high).  I was unable to determine how to operate the WiFi LED, as it
does not appear to be controlled by a GPIO directly.

Recovery:

The default U-boot installation will only boot from flash due to a
missing environment block.  I generated a valid 4KB env block using
U-boot's `fw_setenv` tool and wrote it to flash at 0x30000 using an
external programmer.  After this, it was possible to enter the U-boot
commandline interface and download a new image via TFTP (`tftpboot
81b00000 <image-filename>`), but while I could boot this image
sucessfully (`bootm`), writing it to flash (`cp.linux`) just corrupted
the flash chip.  The sysupgrade file can be written to flash at 0x50000
using an external programmer.

Signed-off-by: Andrew Crawley <acrawley@gmail.com>