media: i2c: Add ov7251_pll_configure()

Rather than having the pll settings hidden inside mode blobs, define
them in structs and use a dedicated function to set them. This makes
it simpler to extend the driver to support other frequencies for both
the external clock and desired link frequency.

Signed-off-by: Daniel Scally <djrscally@gmail.com>
Acked-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>