dt-bindings: leds: add pca955x

This adds the devicetree bindings for the PCA955x I2C LED blinkers.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>