[ARM] 5051/1: define pgtable_t for the !CONFIG_MMU case too

The non-MMU case also needs the type definition of pgtable_t.
So move it out of a CONFIG_MMU conditional section.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>