device property: Unify access to of_node

Historically we have a few variants how we access dev->fwnode
and dev->of_node. Some of the functions during development
gained different versions of the getters. Unify access to of_node
and as a side change slightly refactor ACPI specific branches.

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