ARM: dts: stm32: fix usart 2 & 3 pinconf to wake up with flow control

Modify usart 2 & 3 pins to allow wake up from low power mode while the
hardware flow control is activated. UART RTS pin need to stay configure
in idle mode to receive characters in order to wake up.

Fixes: 842ed898 ("ARM: dts: stm32: add usart2, usart3 and uart7 pins in stm32mp15-pinctrl")

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>