Documentation: mmc: Add mmc-test doc

Add missing documentation for mmc_test.

Reviewed-by: Christian Loehle <christian.loehle@arm.com>
Signed-off-by: Avri Altman <avri.altman@wdc.com>
Link: https://lore.kernel.org/r/20240903133855.3589845-1-avri.altman@wdc.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>