of: make of_node_ktype constant

Since commit ee6d3dd4 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230204-kobj_type-of-v1-1-5910c8ecb7a3@weissschuh.net


Signed-off-by: Rob Herring <robh@kernel.org>