leds: leds-qpnp: Add regulator support for MPP based LEDs

MPP based LEDs may be powered by regulators. Regulator support is
necessary to properly use MPP based LEDs.

Change-Id: Ib3c328f0d5f35be8b171fdd76c75c4e85539c833
Signed-off-by: Chun Zhang <chunz@codeaurora.org>