[media] v4l2-ctrls: create type_ops

Since compound controls can have non-standard types we need to be able to do
type-specific checks etc. In order to make that easy type operations are added.
There are four operations:

- equal: check if two values are equal
- init: initialize a value
- log: log the value
- validate: validate a new value

The v4l2_ctrl struct adds p_new and p_cur unions at the end of the struct.
This union provides a standard way of accessing control types through a pointer,
which greatly simplifies internal control processing.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>