leds-pm8921: blinking buttons for button light notification

Configurable functionality (on/off) that connects Led blinking
events with button-backlight led blinking function.
Using LUT_LOOP mode of the PWM functionality of pm8xxx
to repeat the duty cycles till notification is over.
Taking care that the blinking doesnt happen when touchscreen
is on. Also taking into consideration that the blinking
shouldnt interfere with normal working of the key lights.

The implementation is using synaptics touchscreen's
externalized function to determine whether the screen is on
or off.

Patchset 4: corrections in code style

Change-Id: I9f157ce37129381ccaded472b576218a1223d338