leds: leds-pwm: Add support for blink_set callback

Add blink_set callback to support the led blinking using
the pwm peripheral instead of the software based
blinking supported by the led framework.

Change-Id: I38f7ad5b348639830d11f42dcb960d830c1463c9
Signed-off-by: Kavya Nunna <knunna@codeaurora.org>