powerpc: provide ppc_md.panic() for both ppc32 & ppc64

Allow boards to provide a panic callback on ppc32.  Moved the code to sets
this up into setup-common.c so its shared between ppc32 & ppc64.  Also moved
do_init_bootmem prototype into setup.h.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>