ramips: add support for ELECOM WRH-300CR

ELECOM WRH-300CR is MT7620N based very small Wi-Fi router with 64MiB
DDR2 SDRAM, 16MiB SPI Flash, one fast ethernet port, and (internal but
easy-to-access) UART.

it also has internal USB hub and USB card reader which provide one USB
port, one SD card slot, and one microSD card slot.

Signed-off-by: YuheiOKAWA <tochiro.srchack@gmail.com>
Signed-off-by: FUKAUMI Naoki <naobsd@gmail.com>

SVN-Revision: 49211