of: Support const and non-const use for to_of_node()

Turn to_of_node() into a macro in order to support both const and
non-const use. Additionally make the fwnode argument to is_of_node() const
as well.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Signed-off-by: Rob Herring <robh@kernel.org>