mmc: mmc_test: mention that '0' runs all tests

I had to use the source to determine what I need to write to 'test' so
that all tests are run. Let's mention this explicitly in 'testlist'.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>