ACPI: scan: Add function to fetch dependent of ACPI device

In some ACPI tables we encounter, devices use the _DEP method to assert
a dependence on other ACPI devices as opposed to the OpRegions that the
specification intends.

We need to be able to find those devices "from" the dependee, so add
a callback and a wrapper to walk over the acpi_dep_list and return
the dependent ACPI device.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>