leds: TI LMU: Add common code for TI LMU devices

Create a TI LMU common framework for TI LMU devices that share
common features.

Currently the runtime ramp and brightness setting have
been identified as common features with common register settings.

This work is derived from Milo Kims TI LMU MFD code.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>