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

Documentation/driver-api/mmc/index.rst

@@ -10,4 +10,5 @@ MMC/SD/SDIO card support
    mmc-dev-attrs
    mmc-dev-parts
    mmc-async-req
+   mmc-test
    mmc-tools