media: ccs-pll: Document the structs in the header as well as the function

The CCS pll is used by the CCS driver at the moment, but documenting the
interface makes sense. It's non-trivial and the calculator could be used
elsewhere.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>