ACPI: bus: Introduce acpi_dev_for_each_child()

Introduce a wrapper around device_for_each_child() to iterate over
the children of a given ACPI device object.

This function will be used in subsequent change sets.

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