[media] omap3isp: Calculate vpclk_div for CSI-2

The video port clock is l3_ick divided by vpclk_div. This clock must be high
enough for the external pixel rate. The video port requires two clock cycles
to process a pixel.

Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>