Clean up and reorganise the mmc_test driver so that it (hopefully)
is easier to extend with more complex tests.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

We can only perform the tests on MMC and SD cards, so avoid binding
to any other type.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

Add the ability to run just a single test case by writing the test
case number into the sysfs "test" file.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>

A dummy driver that performs a series of requests that are often mis-
handled by host drivers.
Signed-off-by: NPierre Ossman <drzeus@drzeus.cx>