ARM: pgtable: provide get_top_pte() to complement set_top_pte()

Provide get_top_pte() to complement set_top_pte(), moving the only
users of TOP_PTE to arch/arm/mm/mm.h.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>