This patch improves the backend property, it can be either a phandle or a
of_path. During probe() of the state driver the backend property is
dereferenced and the resulting of_path is saved in the state context. In a
later patch it will be used to generate a phandle reference to the backend
during DT fixup.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>