dt-bindings: bus: Add binding for the Allwinner MBUS controller

The MBUS controller drives the MBUS that other devices in the SoC will
use to perform DMA. It also has a register interface that allows to
monitor and control the bandwidth and priorities for masters on that
bus.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>