media: uapi/linux/cec.h: add defines for the CEC UI Command Operand

These defines were never added to this CEC header, likely due
to laziness on the part of the original author, i.e. me.

But it is useful to have them, so add them.

Also update the cec.h.rst.exceptions file to avoid errors when
building the documentation.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>