dt-bindings: regulator: gdsc-regulator: add bus management bindings

Add definitions for a set of device tree properties which can be
used to manage bus bandwidth votes at runtime.  Such voting is
needed on some SoCs to ensure that the GDSC control register is
accessible over the bus.

Change-Id: If8a525b69bde049da43679fe7bbcac215786415a
Signed-off-by: David Collins <collinsd@codeaurora.org>