[media] media: ov7670: add possibility to bypass pll for ov7675

For a frame rate of 30 fps a pixclk of 24MHz is needed. For those
cases where the ov7670 has a clean 24MHz input (xvclk) the PLL
can be bypassed.
This will result in a value of clkrc of 1, which means that in practice
pixclk = xvclk (input clock)

Acked-by: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Javier Martin <javier.martin@vista-silicon.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>