mmc: mmc-test: add eMMC hardware reset test

MMC core provides a checking function that checks if the reset
has happended.  Add a test to use that function.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Chris Ball <cjb@laptop.org>
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>