mfd: Enable Device Tree support for the db8500-prcmu

This patch will enable probing to occur during a Device Tree enabled
boot. The IRQ base is expected to be located in and will be fetched
from the DT itself. We also prevent any of the db8500 regulators
from being registered here, as they will be enabled via DT instead.

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