drm/vc4: add iec958 controls to vc4_hdmi

Although vc4 get an IEC958 formatted stream passed in from userspace
the driver needs the info from the channel status bits to properly
set up the hardware, eg for HBR passthrough.

Add iec958 controls so the channel status bits can be passed in
from userspace.

Signed-off-by: Matthias Reichl <hias@horus.com>