MIPS: BCM47XX: add EARLY_PRINTK_8250 support

The BCM47xx SoCs have a 8250 serial compatible console at address
0x18000300 and an other at 0x18000400. On most devices 0x18000300 is
wired to some pins on the board, we should use that.
This is the smae for the AI (bcma) and the SB (ssb) bus, this is some
offset on the chip common core.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: John Crispin <blogic@openwrt.org>
Patchwork: http://patchwork.linux-mips.org/patch/5889/