ARM: dts: stm32: Add alternate pinmux for sai2b on stm32mp15

Add another option for the SAI2B pins.

This is used on the Phycore STM32MP1.

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