dt-bindings: serial: add compatible for stm32h7

Introduce new compatibles for "st,stm32h7-usart" and "st,stm32h7-uart".
This new compatible allow to use optional wake-up interrupt.

Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Bich Hemon <bich.hemon@st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>