camss: Use optimal clock rates

Get the current pixel clock rate of the sensor and calculate clock rates
on CSIPHY, CSID and VFE to use the lowest possible.

If the currnet pixel clock rate of the sensor cannot be read then use
the highest possible. This case covers also the CSID test generator
usage.

Signed-off-by: Todor Tomov <todor.tomov@linaro.org>