leds: qpnp-wled: add stepper algorithm using brightness map

Add stepper algorithm support with dynamically calculated step
size or delay based on the brightness level change using the
brightness map table.

To help with running the algorithm efficiently, use a separate
workqueue with high priority to process the brightness levels.

Change-Id: Iea2a8da73b6bee3eaa7b28a12fd82c2a1507db99
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>