ACPI / processor: use ACPI_COMPANION() to get ACPI device

Use ACPI_COMPANION() to get an ACPI device instead of
acpi_bus_get_device() in the processor driver.

Signed-off-by: Lan Tianyu <tianyu.lan@intel.com>
[rjw: Changelog]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>