In order to establish required properties based on the selected
bus type, make the 'bus-type' property mandatory. As this change
documents an endpoint property, also document the 'remote-endpoint'
one now that the 'endpoint' schema has been expanded.

Binary compatibility with existing DTB is kept as the driver does not
enforce the property to be present, and shall fall-back to default
parallel bus configuration, which was the only supported bus type, if
the property is not specified.
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-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>