ARM: dts: stm32: enable sdmmc2 node for stm32mp157c-ed1 board

On STM32MP157C-ED1, the eMMC is connected on instance 2 of SDMMC
peripheral. The sdmmc2 node is then added in the board DT file, as well as
the pins nodes in the pinctrl file.

Signed-off-by: Ludovic Barre <ludovic.barre@st.com>
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>