video: add LINK_FREQ ctrl and a helper to retrieve it

Add a ctrl VIDEO_CID_LINK_FREQ to indicate to a sink
the frequency at which a device streams data over
CSI2. Since not all source device currently provide the
LINK_FREQ control, add a helper function to retrieve
it, with a fall-back by doing an approximate via the
PIXEL_RATE control.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>