feat(st-sdmmc2): make reset property optional

Although not recommended, the reset property could be made optional.
This way the driver will probe even if no reset property is provided
in an sdmmc node in DT. This reset is already optional in Linux.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: I6e63ff00118d9497f505d6379982334dd62686ca