ARM: dts: stm32: fix AV96 board SAI2 pin muxing on stm32mp15

Fix SAI2A and SAI2B pin muxings for AV96 board on STM32MP15.
Change sai2a-4 & sai2a-5 to sai2a-2 & sai2a-2.
Change sai2a-4 & sai2a-sleep-5 to sai2b-2 & sai2b-sleep-2

Fixes: dcf185ca ("ARM: dts: stm32: Add alternate pinmux for SAI2 pins on stm32mp15")

Signed-off-by: Olivier Moysan <olivier.moysan@foss.st.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>