When running benchmarks on a system and inspecting the effect of
performance related changes it can be useful to know in a
simple way how long it takes for UFS requests to complete.
Add statistics for UFS requests completion times and measure
minimum, maximum and average completion times for all requests
and also according to request type: read, write and flush.
Change-Id: I85639d629939e3ae3bba6de065ceadae0da3dc99
Signed-off-by: Gilad Broner <gbroner@codeaurora.org>