ACPI: utils: Add acpi_dev_uid_to_integer() helper to get _UID as integer

Some users interpret _UID only as integer and for them it's easier to
have an integer representation of _UID. Add respective helper for that.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>