[media] smiapp-pll: Add a few comments to PLL calculation

The PLL calculation heuristics is rather complicated and and is often
difficult to understand to its original author.

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