dt-bindings: gpu: add bus clock for Mali Midgard GPUs

Some SoCs adds a bus clock gate to the Mali Midgard GPU.

Add the binding for the bus clock.

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Clément Péron <peron.clem@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190521161102.29620-4-peron.clem@gmail.com