sh: allow earlyprintk baud rate to be set via command line

This allows the baud rate for earlyprintk for sh4 without the
standard BIOS to be set via the command line. This uses the same
format as i386 and x86_64, which is:

	earlyprintk=serial,ttySC1,38400

The second parameter (ttySC1 above) is usually the console device
name or the io address of the serial port. I allow that to be
specified but ignore it in order to keep the format the same as
i386/x86_64.

Signed-off-by: Jamie Lenehan <lenehan@twibble.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>