dt-bindings: opp: Convert ti-omap5-opp-supply to json schema

Rename ti-omap5-opp-supply to be bit more generic omap-opp-supply and
convert the free text binding to json-schema.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Dhruva Gole <d-gole@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>