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>

Documentation/devicetree/bindings/leds/leds-pca955x.txt

@@ -9,6 +9,7 @@ Required properties:
 	"nxp,pca9550"
 	"nxp,pca9551"
 	"nxp,pca9552"
+	"ibm,pca9552"
 	"nxp,pca9553"
 - #address-cells: must be 1
 - #size-cells: must be 0