mmc: Add profiling code to measure performance at MMC layers.

Profiling code is added to measure read, write times for
the MMC requests at various MMC layers. Profiling is done
at the MMC queue and at the driver level. This information
can be viewed through a sysfs entry called perf.

Change-Id: I7c65bfe25a1f7774e3a9abf1f9539e690b3718ec
Signed-off-by: Aparna Mallavarapu <aparnam@qualcomm.com>