test: spl: Add functions to create filesystems

Add some functions for creating fat/ext2 filesystems with a single file and
a test for them. Filesystems require block devices, and it is easiest to
just use MMC for this. To get an MMC, we must also pull in the test device
tree. SPL_TIMER is necessary for SPL_MMC, perhaps because it uses a timeout.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>