ARM: mach-shmobile: kzm9g: enable SMP boot

Update the KZM9G defconfig and the code in platsmp.c to support SMP on
the sh73a0 based KZM9G board. Also fix up the earlyprintk setting that
was previously incorrect.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>