mmc_test: add performance tests

mmc_test provides tests aimed at testing SD/MMC hosts.  This patch adds
performance tests.

It is advantageous to have performance tests in a kernel
module like mmc_test for the following reasons:
	- transfer times can be measured very accurately
	- arbitrarily large transfers are possible
	- the effect of contiguous vs scattered pages
	can be determined

The new tests are:

	23. Best-case read performance
	24. Best-case write performance
	25. Best-case read performance into scattered pages
	26. Best-case write performance from scattered pages
	27. Single read performance by transfer size
	28. Single write performance by transfer size
	29. Single trim performance by transfer size
	30. Consecutive read performance by transfer size
	31. Consecutive write performance by transfer size
	32. Consecutive trim performance by transfer size

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Cc: <linux-mmc@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>