ACPI: processor: introduce acpi_processor_alloc_pdc()

acpi_processor_init_pdc() isn't really doing anything interesting
with the struct acpi_processor * parameter. Its real job is to allocate
the buffer for the _PDC bits.

So rename the function to acpi_processor_alloc_pdc(), and just return
the struct acpi_object_list * it's supposed to allocate.

Signed-off-by: Alex Chiang <achiang@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>