Rename static get_cpu_id() to acpi_get_cpuid() and export it.

This change also gives us an opportunity to remove the
#ifndef CONFIG_SMP from processor_driver.c and into a header file
where it properly belongs.
Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>