mfd: Initialise the AB8500 driver at core_initcall time

The AB8500 is soon to have its own IRQ domain. For this to be useful
the driver needs to be initialised earlier in the boot sequence. Here
we move initialisation forward from arch_initcall to core_initcall time.

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>