ARM: dts: stm32: Add usart2_pins on stm32h743

Add usart2 pins definition in order to add usart2 support
dedicated for console output on stm32h743i-disco board.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>