mfd: Initialise the DB8500 PRCMU driver at core_initcall time

Now the AB8500 has its own IRQ domain it needs to be initialised earlier
in the boot sequence. As the AB8500 relies on the DB8500 PRCMU we need to
reflect this change for the PRCMU driver too.

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