boards: arm: nucleo_h723zg board enabling usart2 This enables defines the usart2 on PD5, PD6 pins of the nucleo stm32h723zg target board. Pins are available on the CN9 pin6 & 4 of the MB1364 nucleo144 Signed-off-by: Francois Ramu <francois.ramu@st.com>