power: Add option to log time spent in suspend

Prints the time spent in suspend in the kernel log, and
keeps statistics on the time spent in suspend in
/sys/kernel/debug/suspend_time

Change-Id: Ia6b9ebe4baa0f7f5cd211c6a4f7e813aefd3fa1d
Signed-off-by: Colin Cross <ccross@android.com>
Signed-off-by: Todd Poynor <toddpoynor@google.com>