ARM: dts: stm32: update eMMC configuration for stm32mp157c-ev1

Update the sdmmc2 node for eMMC support on eval board stm32mp157c-ev1.
- update slew-rate for pin configuration
- update "vqmmc-supply"
- remove "st,sig-dir"
- add mandatory "pinctrl-names"
- add "mmc-ddr-3_3v"

This patch solve the eMMC detection issue for command "mmc dev 1".

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>