ARM: dts: stm32: add st,stm32-sdmmc2 compatible on stm32mp151

To align with bootloaders device tree files, and thanks to what was
added in yaml file [1], the compatible property for sdmmc nodes is
updated with "st,stm32-sdmmc2" string.

[1] commit 552bc464 ("dt-bindings: mmc: mmci: Add st,stm32-sdmmc2
    compatible")

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>