dt-bindings: leds: pca955x: Add IBM implementation compatible string

IBM created an implementation of the PCA9552 on a PIC16F
microcontroller. Document the new compatible string for this device.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>