mfd: add DB5500 PRCMU driver

This adds the DB5500 PRCMU driver. Right now this one is pretty
restricted in functionality, exposing a simple interface to send
I2C messages.

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