[ARM] Introduce new PTE memory type bits

Provide L_PTE_MT_xxx definitions to describe the memory types that we
use in Linux/ARM.  These definitions are carefully picked such that:

1. their LSBs match what is required for pre-ARMv6 CPUs.
2. they all have a unique encoding, including after modification
   by build_mem_type_table() (the result being that some have more
   than one combination.)

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