leds: lp50xx: Add the LP50XX family of the RGB LED driver

Introduce the LP5036/30/24/18/12/9 RGB LED driver.
The difference in these parts are the number of
LED outputs where the:

LP5036 can control 36 LEDs
LP5030 can control 30 LEDs
LP5024 can control 24 LEDs
LP5018 can control 18 LEDs
LP5012 can control 12 LEDs
LP5009 can control 9 LEDs

The device has the ability to group LED outputs into control banks
so that multiple LED banks can be controlled with the same mixing and
brightness. Or the LED outputs can also be controlled independently.

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