dt-bindings: qcom_spmi: Document SAW support

Document the DT bindings for the SAW regulators.

The saw-leader is the only property that is configurable in DT.

The saw-slave property allows ganging (grouping) of
several regulators so that their outputs can be combined.

Signed-off-by: Ilia Lin <ilialin@codeaurora.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>