drivers: disk: sdmmc_stm32: Add API to retrieve SD card CID register

Introduce a new API function in the sdmmc_stm32 driver that allows
applications to access the Card Identification (CID) register of the
SD/MMC card. This functionality, already available in the SPI-based SD
and MMC subsystems, was previously missing from the STM32 SDMMC driver.

This enhancement enables use cases such as verifying the correct SD card
during manufacturing, ensuring that OEMs use the specified SD card, and
preventing mismatches.

Signed-off-by: Arthur Gay <arthur.gay@marshmallow.kids>