led: add TI LP5562 LED driver

led: add TI LP5562 LED driver

Driver for the TI LP5562 4 channel LED controller. Supports
independent on/off control of all 4 channels. Supports LED_BLINK on 3
independent channels: blue/green/red. The white channel can blink, but
shares the blue channel blink rate.

Heavily based on patch originally from Doug Zobel [1].

I have modified it so it matches the DT bindings in the linux tree,
and also follows the linux driver implementation more closely. This
should address Tom's concerns, and also matches my goal of making the
U-Boot driver work with our existing .dts which is known to work in
linux.

As our boards only have the R,G,B outputs connected, I have not
actually tested how the white channel behaves, but the R,G,B work
exactly as expected.

[1] https://lore.kernel.org/u-boot/1547150757-1561-1-git-send-email-douglas.zobel@climate.com/

Cc: Doug Zobel <douglas.zobel@climate.com>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>