V4L/DVB (11030): soc-camera: add board hook to specify the buswidth for camera sensors

Camera sensors have a native bus width say support, but on some
boards not all sensor data lines are connected to the image
interface and thus support a different bus width than the sensors
native one. Some boards even have a bus driver which dynamically
switches between different bus widths with a GPIO.

This patch adds a hook which board code can use to support different
bus widths.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>