regulator: Convert stm32-pwr regulator to json-schema

Convert the stm32-pwr regulator binding to DT schema format using
json-schema.

Signed-off-by: Pascal Paillet <p.paillet@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191205161359.20755-1-p.paillet@st.com


Signed-off-by: Mark Brown <broonie@kernel.org>