[POWERPC] Start factoring pgtable-ppc32.h and pgtable-ppc64.h

This factors some things defined in both pgtable-ppc32.h and
pgtable-ppc64.h into the common part of asm-powerpc/pgtable.h.  These
are all things which have essentially identical definitions, and which
by their nature are very unlikely ever to need different definitions
in the two cases.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>