leds: qpnp-wled: add support for brightness mapping

Currently, WLED driver is being passed with a 12 bit brightness
level by the clients and they're set in the hardware registers.
Add support for brightness mapping by providing a 256 entry table
where each entry can take a 12 bit brightness level value. This
will be mapped internally for controlling different brightness
regions in an user preferred way.

Change-Id: I7adf97918a311bf54b02b383defeb59685aa0d00
Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>