dt-bindings: power: Convert imx gpcv2 to json-schema

Convert the i.MX GPCv2 binding to DT schema format using json-schema

Example is updated based on latest DT file and consumer's example is
removed since it is NOT that useful.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Rob Herring <robh@kernel.org>