ARM: dts: stm32: Add new pinmux for sdmmc1_b4 on stm32mp15

Add another option for the SDMMC_B4 pins.
It is almost identical to sdmmc1_b4_pins_a but the SDMMC1_D2 pin.

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>