[media] cec-funcs.h: static inlines to pack/unpack CEC messages

This public header contains static inlines to pack and unpack CEC
messages. It is for use in both the kernel and in userspace.

Since the CEC framework will initially be in staging this header is
not yet in include/uapi. Once the framework is moved out of staging
this header should be moved to uapi at the same time.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>