dt-bindings: regulator: Convert stm32 vrefbuf bindings to json-schema

Convert the STM32 regulator vrefbuf binding to DT schema format using json-schema

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
CC: Fabrice Gasnier <fabrice.gasnier@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20191122104536.20283-2-benjamin.gaignard@st.com
Signed-off-by: Mark Brown <broonie@kernel.org>