dt-bindings: leds: Add bindings for lltc,lt3593

This patch adds the bindings document for LT3593 LED drivers.
The binding is kept consistent with other LED driver bindings in that it
stores all the LED-specific properties in its own subnode. As the hardware
only supports one channel, there can consequently only be one sub-node.

Signed-off-by: Daniel Mack <daniel@zonque.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>