ACPI / property: Document usage rules for _DSD properties

Following some discussions during the Kernel Summit and LPC, document
what can be returned from ACPI _DSD as device properties and when it
is valid to use the special PRP0001 device ID.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviwed-by: Mark Brown <broonie@kernel.org>