ARM: dts: stm32: Create separate pinmux for qspi cs pin in stm32mp15-pinctrl.dtsi

Create a separate pinmux for qspi chip select in stm32mp15-pinctrl.dtsi.
In the case we want to use transfer_one() API to communicate with a SPI
device, chip select signal must be driven individually.

Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>