powerpc: Merge in 64-bit powermac support.

This brings in a lot of changes from arch/ppc64/kernel/pmac_*.c to
arch/powerpc/platforms/powermac/*.c and makes various minor tweaks
elsewhere.  On the powermac we now initialize ppc_md by copying
the whole pmac_md structure into it, which required some changes in
the ordering of initializations of individual fields of it.

Signed-off-by: Paul Mackerras <paulus@samba.org>