ARM: pgtable: move TOP_PTE address definitions to arch/arm/mm/mm.h

Move the TOP_PTE address definitions to one central place so that it's
easy to discover what they're being used for.  This helps to ensure
that there are no overlaps.

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