mfd: Register the ab8500 from db8500-prcmu using the MFD API

Hierarchically, the AB8500 is a child of the DB8500 PRCMU. So now that
Device Tree is being used and MFD core code is Device Tree aware, we
can simply register DB8500 PRCMU from Device Tree in the normal way
then allow the DB8500 PRCMU driver to register the AB8500 as a simple
MFD device at probe time.

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