ARM: dts: stm32: Add idle/sleep pinmux for USART3 on stm32mp15

Add idle and sleep mux option for the USART3 pins_a.

This is used on the Phycore STM32MP1.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>