ARM: Introduce arch_read_hardware_id

Moving towards device tree and arm single binary refering to
machine descriptor name for hardware id information under
/proc/cpuinfo is not suitable for certain soc vendors. Add a
hook for soc vendors to supply a per-soc hardware read method.

Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>
(cherry picked from commit eab25833f0d433efd5636ceed506b677438b7cc2)
Signed-off-by: Josh Cartwright <joshc@codeaurora.org>