ARM/ARM64: 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.

Change-Id: Ifcccdffa3c0e1e8b5f96837eb1c023e468d4c287
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>