mfd: ab8500-core: Apply the AB8500 CODEC's compatible string to its MFD cell

Provide a compatible string for the AB8500 CODEC to aid in
configuration property look-up from its associated Device Tree
node.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>