mediatke: add support for elecom-wrc-2533gent

This commit adds support for the MT7622-based Elecom WRC-2533gent router,
with spi-nand storage and 512MB RAM.

The device has the following specifications:

* MT7622 (arm64 dual-core)
* 512MB RAM (DDR3)
* 4GB storage (spi-nand)
* 5x 1Gbps Ethernet (RTL8337C switch)
* 1x UART header
* 1x USB 3.0 port
* 5x LEDs
* 1x reset button
* 1x WPS button
* 1x slider switch
* 1x DC jack for main power (12V)

The following has been tested and is working:
* Ethernet switch
* 2.4g and 5g wifi
* USB 3.0 port
* sysupgrade
* buttons/leds

Not working:
* bluetooth firmware does not load even though it is present int he rootfs

Signed-off-by: John Crispin <john@phrozen.org>