powerpc/32: Setup OF properties for kdump

Refactor the setting of kdump OF properties, moving the common code
from machine_kexec_64.c to machine_kexec.c where it can be used on
both ppc64 and ppc32.  This will be needed for kdump to work on ppc32
platforms.

Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>