ARM: mach-shmobile: early console support using earlyprintk

Add support for early console to the SH-Mobile boards.

Simply perform an early platform probe for "earlyprintk"
to trigger the early serial console code in sh-sci.c.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>