ACPI: processor: remove .start() method

This patch folds the .start() method into .add().

acpi_processor_start() is always called immediately after
acpi_processor_add(), so there's really no point in having them be
separate methods.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
CC: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
CC: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>