dts: bindings: serial: Add common bindings for STM32 serial driver.

There are 3 bindings for STM32 serial driver:
st,stm32-uart.yaml
st,stm32-usart.yaml
st,stm32-lpuart.yaml
Add a common st,stm32-uart-base.yaml that would be included by these
3 bindings an would group common properties.


Signed-off-by: TLIG Dhaou <dhaou.tlig-ext@st.com>