device property: constify property arrays values

Data that is fed into property arrays should not be modified, so let's mark
relevant pointers as const. This will allow us making source arrays as
const/__initconst.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>