mfd: ab8500-core: Add of_compatilbe property for ab8500-usb

Add of_comptabile string to the ab8500-usb cell to allow the driver to
grab properties from device-tree when available.

Signed-off-by: Fabio Baltieri <[e-mail]>
Signed-off-by: Samuel Ortiz <[e-mail]>

drivers/mfd/ab8500-core.c

···
 	},
 	{
 		.name = "ab8500-usb",
+		.of_compatible = "stericsson,ab8500-usb",
 		.num_resources = ARRAY_SIZE(ab8500_usb_resources),
 		.resources = ab8500_usb_resources,
 	},