media: linux/cec.h: add pin monitoring API support

Add support for low-level CEC pin monitoring. This adds a new monitor
mode, a new capability and two new events.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Reviewed-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>