dt-bindings: rng: Convert stm32 RNG bindings to json-schema

Convert the STM32 RNG binding to DT schema format using json-schema
Remove interrupt from the json-schema because it is not used by the driver.

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>