dt-bindings: i2c: i2c-stm32f7: allow clock-frequency range

For STM32F7, STM32H7 and STM32MP1 SoCs, if timing parameters
match, the bus clock frequency can be from 1Hz to 1MHz.

Signed-off-by: Alain Volmat <alain.volmat@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>