mfd: Initialize DB8500 PRCMU regs

Some clocks may be force enabled when we probe the
driver, but they need to be turned off by default so
we have a known state. We call this the register
initialization function if we need more stuff in
there in the future.

Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>