media: cec: add support for Absolute Volume Control

Add support for this new CEC message. This was added in HDMI 2.1a.

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

media: include/uapi/linux/cec.h: typo: SATERDAY -> SATURDAY

Fix typo in a define: CEC_OP_REC_SEQ_SATERDAY -> CEC_OP_REC_SEQ_SATURDAY

This isn't used yet in actual applications to the best of my knowledge,
and it certainly doesn't break the ABI since the value doesn't change.

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

media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee3862 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>