mfd: Add initial db8500 prcmu register access api

This patch adds an initial PRCMU register access API, which
for now should only be used for a very limited set of registers.

The idea about this API is that we split the PRCMU driver in
one part that deals with interaction with the PRCMU firmware
and one part that simply provide write accessors in the PRCMU
register range. The latter are just a collection of registers
exposed in the PRCMU register range for various purposes and
not related to the PRCMU firmware.

Currently we support some limited GPIO, SPI and UART settings
through this API.

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>