[ARM] mm 1: Combine mem_type domain into prot_* at init time

Rather than combining the domain for a particular memory type with
the protection information each time we want to use it, do so when
we fix up the mem_type array at initialisation time.

Rename struct mem_types to be mem_type - each structure is one
memory type description, not several.

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