media: rcar-csi2: Switch to using fwnode instead of OF

Use the fwnode_graph_get_endpoint_by_id() interface instead of
of_graph_get_endpoint_by_regs() to fetch the fwnode. This saves
translating between a device_node and fwnode_handle.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo+renesas@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>