dt-bindings: interconnect: sunxi: Add R40 MBUS compatible

The R40 MBUS compatible was introduced recently but it was never
documented.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Reviewed-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://lore.kernel.org/r/20210901091852.479202-26-maxime@cerno.tech