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.

Change-Id: I8d6f77dbb8037c943d58e0d39fcaa9b54366650e
Signed-off-by: Abhimanyu Kapur <abhimany@codeaurora.org>