dt-bindings: media: clarify stm32 csi & simplify example

Clarify the description of the stm32 CSI by mentioning CSI-2 and D-PHY.
Remove the bus-type property from the example since this CSI has a D-PHY,
which is the only bus-type option, making this property redundant.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>