MBUS clock will be referenced in MBUS controller node.

Export it.

Acked-by: Maxime Ripard <mripard@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jernej Skrabec <jernej.skrabec@siol.net>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>