pinctrl/nomadik: make independent of prcmu driver

Currently there are some unnecessary criss-cross
dependencies between the PRCMU driver in MFD and a lot of
other drivers, mainly because other drivers need to poke
around in the PRCM register range.

In cases like this there are actually just a few select
registers that the pinctrl driver need to read/modify/write,
and it turns out that no other driver is actually using
these registers, so there are no concurrency issues
whatsoever.

So: don't let the location of the register range complicate
things, just poke into these registers directly and skip
a layer of indirection.

Take this opportunity to add kerneldoc to the pinctrl
state container.

Cc: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>