drm/vc4: hdmi: Support BCM2711 CEC interrupt setup

The HDMI controller found in the BCM2711 has an external interrupt
controller for the CEC and hotplug interrupt shared between the two
instances.

Let's add a variant flag to register a single interrupt handler and
deals with the interrupt handler setup, or two interrupt handlers
relying on an external irqchip.

Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Dave Stevenson <dave.stevenson@raspberrypi.com>
Acked-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Tested-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Link: https://patchwork.freedesktop.org/patch/msgid/20210111142309.193441-11-maxime@cerno.tech