dts: bindings: Add optional timings capability

By default the I2C timing (I2C_TIMINGR register) of I2C V2 peripheral is
computed depending on clocks at runtime with relatively low precision.

This optional property allows to provide a list of pre-computed values
(typically computed from STM32CubeMX SW tool) instead of using runtime
algorithm.

The precomputed value is only valid for a given clock configuration and
I2C speed setting, so this valid configuration is provided with each
value entry.

Signed-off-by: Laurent Meunier <laurent.meunier@st.com>