drm/vc4: hdmi: Introduce a CEC clock

drm/vc4: hdmi: Introduce a CEC clock

While the BCM2835 had the CEC clock derived from the HSM clock, the
BCM2711 has a dedicated parent clock for it.

Let's introduce a separate clock for it so that we can handle both
cases.

Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111142309.193441-9-maxime@cerno.tech
(cherry picked from commit cd7f016c93b697dea79a884162cbf7311283fe12)
Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>