media: ar0521: Rework PLL computation

Rework the PLL computation procedure to take into account the currently
configured format bpp and the number of data lanes.

Comment the PLL configuration procedure with information provided by the
sensor chip manual and remove the hardcoded divider from the pixel clock
calculation.

Signed-off-by: Jacopo Mondi <jacopo@jmondi.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>