ACPI / bus: Move duplicate code to a separate new function

After merging commit 712e960f (ACPI / PM: Attach ACPI power
domain only once) with commit 1dcc3d33 (ACPI / bus: Move ACPI
bus type registration) there is some duplicate code in
acpi_device_is_first_physical_node() and acpi_companion_match()
that can be moved to a separate routine and called from both
places.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Reviewed-by: Hanjun Guo <hanjun.guo@linaro.org>