dt-bindings: i2c: i2c-st: convert to DT schema

Convert i2c-st.txt into st,sti-i2c.yaml for the i2c-st driver.

Signed-off-by: Alain Volmat <avolmat@me.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>