dt-bindings: leds: Add binding for spi-byte LED.

This patch adds the binding documentation for a simple SPI based LED
controller which use only one byte for setting the brightness.

Signed-off-by: Christian Mauderer <oss@c-mauderer.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>