dt-bindings: opp: Introduce ti-opp-supply bindings

Document the devicetree bindings that describe Texas Instruments
opp-supply which allow a platform to describe multiple regulators and
additional information, such as registers containing data needed to
program aforementioned regulators.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>