dt-bindings: i2c: maxim,max96712: Require setting bus-type property

The MAX96712 can support both a CSI-2 C-PHY and D-PHY bus. The initial
staging driver however only supported D-PHY and the bus-type property
was left optional.

In preparation for adding C-PHY support to the staging driver make the
bus-type property mandatory as it is needed to select the correct PHY
mode. Without the bus-type property present, the driver falls-back to
D-PHY mode, so the change is functionally backward compatible with old
DTS files lacking the property.

The only in-tree DTS file (renesas/r8a779a0-falcon-csi-dsi.dtsi) that
lacked the property uses D-PHY and have been updated.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>