kernel/power: add pollable sysfs entries for touch events

Provide userspace with a pollable sysfs entry that it can poll
on waiting for touch event notifiction. Once it
is woken up, the userspace can take action to change the
power/performance characteristics of the device for the duration
of the event.

Change-Id: Iab84f729f2ea3a5352c4e5173f78544023a276c1
Signed-off-by: Amar Singhal <asinghal@codeaurora.org>