mfd: Enable Device Tree for ab8500-core driver

This patch will allow the ab8500-core driver to be probed and set up
when booting when Device Tree is enabled. This includes platform ID
look-up which identifies the machine it is currently running on. If
we are undergoing a DT enabled boot, we will refuse to setup each of
the other ab8500-* devices, as they will be probed individually by DT.

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