mmc: mmc_test: add tests to measure large sequential I/O performance

Add two large sequential I/O performance tests:
        35. Large sequential read into scattered pages
        36. Large sequential write from scattered pages

The tests measure transfer times for 10MiB, 100MiB, 1000MiB.

Signed-off-by: Adrian Hunter <adrian.hunter@nokia.com>
Signed-off-by: Chris Ball <cjb@laptop.org>