mfd: Add db8500-pcmu watchdog accessor functions for watchdog

This implements the watchdog accessor functions for the DB8500
PRCMU, making it possible to implement the watchdog driver.

Signed-off-by: Jonas Aberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>