dt-bindings: drm/bridge: anx7625: Revert DPI support

Revert DPI support from binding.

DPI support relies on the bus-type enum which does not yet support
Mipi DPI, since no v4l2_fwnode_bus_type has been defined for this
bus type.

When DPI for anx7625 was initially added, it assumed that
V4L2_FWNODE_BUS_TYPE_PARALLEL was the correct bus type for
representing DPI, which it is not.

In order to prevent adding this mis-usage to the ABI, let's revert
the support.

Signed-off-by: Robert Foss <robert.foss@linaro.org>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>