mfd: Enable DT probing of the DB8500 PRCMU

This patch adds the correct compatible string for use during Device Tree
population. Without it the DB8500 PRCMU will not be probed.

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