test: drivers: disk: disk_access: add STM32 specific testcase

STM32 devices with an SDMMC driver will still need to be tested, but
they do not use the SDHC framework for their disk driver. Therefore, add
a special testcase to validate disk access support for devices with the
STM32 specific SDMMC DT compatible enabled.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>