Documentation: gpu: vc4: Add KUnit Tests Section

Now that we have VC4-specific tests in place, let's document them
properly.

Reviewed-by: Maíra Canal <mcanal@igalia.com>
Link: https://lore.kernel.org/r/20221123-rpi-kunit-tests-v3-20-4615a663a84a@cerno.tech
Signed-off-by: Maxime Ripard <maxime@cerno.tech>

Documentation/gpu/vc4.rst

@@ -54,6 +54,22 @@ VEC (Composite TV out) encoder
 .. kernel-doc:: drivers/gpu/drm/vc4/vc4_vec.c
    :doc: VC4 SDTV module
 
+KUnit Tests
+===========
+
+The VC4 Driver uses KUnit to perform driver-specific unit and
+integration tests.
+
+These tests are using a mock driver and can be ran using the
+command::
+	./tools/testing/kunit/kunit.py run \
+		--kunitconfig=drivers/gpu/drm/vc4/tests/.kunitconfig \
+		--cross_compile aarch64-linux-gnu- --arch arm64
+
+Parts of the driver that are currently covered by tests are:
+ * The HVS to PixelValve dynamic FIFO assignment, for the BCM2835-7
+   and BCM2711.
+
 Memory Management and 3D Command Submission
 ===========================================