mmc: Enable eMMC unit-tests

Enable the compilation of eMMC4.5 unit-tests, required by APT team.
This will allow the APT team to test the storage activity on released
builds.
The storage tests are disabled in normal operation and in order to
activate them a test I/O scheduler should be chosen and the test should
be triggered via debugfs. Therefore they have no effect on normal eMMC
driver operation.

Change-Id: I179c567f67cc8fab9ed1edab8246483de18bc76a
Signed-off-by: Maya Erez <merez@codeaurora.org>