dt-bindings: media: qcom,x1e80100-camss: Add optional bus-type property

Since CSIPHY IP on modern Qualcomm SoCs supports D-PHY and C-PHY
interfaces, it might be necessary to specify it explicitly for some
particular devices.

Signed-off-by: Vladimir Zapolskiy <vladimir.zapolskiy@linaro.org>
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>