device property: refactor built-in properties support

Instead of using the type and nval fields we will use length (in bytes) of the
value. The sanity check is done in the accessors.

The built-in property accessors are split in the same way such as device tree.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>