mmc: mmc_test: add test for non-blocking transfers

Add four tests for read and write performance per
different transfer size, 4k to 4M.
 * Read using blocking mmc request
 * Read using non-blocking mmc request
 * Write using blocking mmc request
 * Write using non-blocking mmc request

The host driver must support pre_req() and post_req()
in order to run the non-blocking test cases.

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>