leds: leds-qpnp: Support vin control for MPP LEDs

MPP can support input voltages from vin0 to vin3. Parse
this data from Device Tree and program appropriate MPP
registers. Also add support for minimum brightness that
MPP LED can support.

Change-Id: Ieb4f924321f224b1915539befe33abf3b03f6ac3
Signed-off-by: Chun Zhang <chunz@codeaurora.org>
Signed-off-by: Mohan Pallaka <mpallaka@codeaurora.org>