dt-bindings: media: camss: Restrict bus-type property

The CSIPHY of Qualcomm SoCs support both D-PHY and C-PHY standards for
CSI-2, but not any others so restrict the bus-type property describing
this to the supported values.

The only exception here is MSM8916 which only supports D-PHY. C-PHY was
introduced with newer SoCs.

Do note, that currently the Linux driver only supports D-PHY.

Signed-off-by: Luca Weiss <luca.weiss@fairphone.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Bryan O'Donoghue <bod@kernel.org>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>