mmc: mmc_test: test to measure how sg_len affect performance

Add a test that measures how the mmc bandwidth depends on the numbers of
sg elements in the sg list. The transfer size if fixed and sg length goes
from a few up to 512. The purpose is to measure overhead caused by
multiple sg elements.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Venkatraman S <svenkatr@ti.com>
Tested-by: Sourav Poddar <sourav.poddar@ti.com>
Tested-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>