Android: MMC/UFS IO Latency Histograms.

This patch adds a new sysfs node (latency_hist) and reports
IO (svc time) latency histograms. Disabled by default, can be
enabled by echoing 0 into latency_hist, stats can be cleared
by writing 2 into latency_hist.

Bug: 30677035
Change-Id: Ic6c2ef660aa686e2b32142af4769a3b9e1d0a172
Signed-off-by: Mohan Srinivasan <srmohan@google.com>