ARM: Define CPU part numbers and implementors

Define implementor IDs, part numbers and Xscale architecture versions in
cputype.h.  Also create accessor functions for reading the implementor,
part number, and Xscale architecture versions from the CPUID regiser.

Signed-off-by: Christoffer Dall <c.dall@virtualopensystems.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>