of: Add of_fwnode_handle() to convert device nodes to fwnode_handle

of_fwnode_handle() returns a struct fwnode_handle of the struct
device_node. This may be used on the fwnode property API.

Use a macro instead of a function in order to support const and non-const
arguments.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>