dt-bindings: i2c: i2c-stm32f7: add st,stm32mp15-i2c compatible

Add a new stm32mp15 specific compatible to handle FastMode+
registers which are different on the stm32mp15 compared
to the stm32f7 or stm32h7.

Signed-off-by: Alain Volmat <alain.volmat@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>