ARM: dts: stm32: Add sleep pinmux for SPI1 pins_a on stm32mp15

Add a sleep mux option for the SPI1 pins_a mux.

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>