drm/vc4: Improve the KUnit documentation

The command-line can be expressed using a code-block, and we were
missing which architectures were available.

Suggested-by: Maíra Canal <mcanal@igalia.com>
Reviewed-by: Maíra Canal <mcanal@igalia.com>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20221208094727.2848310-1-maxime@cerno.tech