leds: qti-tri-led: Add breath feature for tri-led

Some PWM devices could output modulated waveforms with a predefined
pattern which can be used for controlling LED breathing behavior. Add
a sysfs parameter to make the LED device can output breathing pattern.

Change-Id: I1e2c60ea3c61bf14d382991b40536d0cdb39006c
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>