MIPS: ralink: make early_printk work on RT2880

RT2880 has a different location for the early serial port.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Gabor Juhos <juhosg@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5170/