PM / Sleep: add knob for printing device resume times

Added a new knob called /sys/power/pm_print_times. Setting it to 1
enables printing of time taken by devices to suspend and resume.
Setting it to 0 disables this printing (unless overridden by
initcall_debug kernel command line option).

Signed-off-by: Sameer Nanda <snanda@chromium.org>
Acked-by: Greg KH <gregkh@linuxfoundation.org>
Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>