[POWERPC] Fix arch/ppc compilation - add typedef for pgtable_t

Commit 2f569afd

 ("CONFIG_HIGHPTE vs.
sub-page page tables.") breaks compilation of arch/ppc since it
introduces the pgtable_t type which was not added to arch/ppc.

This adds the missing typedef.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>