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 <fabio.baltieri@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>