media: dt-bindings: media: st,stm32-dcmi: add support of BT656 bus

Add support of BT656 embedded synchronization bus mode in DCMI driver.
Add "bus-type" property and make it required so that there is no
ambiguity between parallel mode (bus-type=5) and BT656 mode (bus-type=6).
BT656 mode allows to save hardware synchro lines hsync & vsync by replacing
them with synchro codes embedded in data stream, hence hsync-active &
vsync-active properties are useless in this mode.
With DCMI, BT656 bus mode is only compatible with 8 bits width data bus.

Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>