mfd: Add support for TWL6030 PWM

TWL6030 supports PWM (Pulse Width Modulator) which is used
to control charging LED. PWM allows for controlling brightness.
This patch implements the APIs required by leds-pwm driver.

Signed-off-by: Hemanth V <hemanthv@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>