device property: allow to constify properties

There is no reason why statically defined properties should be modifiable,
so let's make device_add_properties() and the rest of pset_*() functions to
take const pointers to properties.

This will allow us to mark properties as const/__initconst at definition
sites.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>