sh-sci: Extend sh-sci driver with early console V2

This is V2 of early serial console support for the sh-sci
driver. The early serial console is using early platform
devices and "earlyprintk". To use this feature the early
platform devices must be broken out to one device per port
and the desired port should be selected on the kernel command
line like: "earlyprintk=sh-sci.N[,baudrate][,keep]"

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