input: sensors: add wake up interface for low power sensor

Add enable_wakeup interface in sensors calss to provide an interface
for user space applications to control the wake up flag. The wake up
flag indicates if the driver works on wake up mode, which means it
can wake up the processor even it is in suspend mode.

Change-Id: I8edf145a190093a4c5384fd948b067b788b1fb1a
Signed-off-by: Oliver Wang <mengmeng@codeaurora.org>