mem= command line option was being ignored in arch/powerpc if we were not
a CONFIG_MULTIPLATFORM (which is handled via prom_init stub). The initial
command line extraction and parsing needed to be moved earlier in the boot
process and have code to actual parse mem= and do something about it.

Also, fixed a compile warning in the file.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>