ACPI / PM: Use ACPI_COMPANION() to get ACPI companions of devices

The ACPI device PM code in device_pm.c uses a special function,
acpi_dev_pm_get_node(), to obtain an ACPI companion object of a given
device.  However, that is not necessary any more after recent changes
that introduced the ACPI_COMPANION() macro serving exactly the same
purpose, but working in a much more straightforward way.  For this
reason, drop acpi_dev_pm_get_node() and use ACPI_COMPANION() instead
of it everywhere.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>