mfd: ab8500-core: Add of_compatible property for ab8500-codec

Add of_compatible string to the ab8500-codec cell to allow the driver to
grab handlers such as regulators from device-tree when available.

Signed-off-by: Fabio Baltieri <fabio.baltieri@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>