ARM: ux500: Provide a link from AB8500 Pinctrl to the PRCMU

The AB8500 Pinctrl driver uses PRCMU register addresses to
control Pinctrl related functions. For this to happen, the
Pinctrl driver needs the PRCMU base to work from. We can do
that using standard Open Firmware (of_*) function calls, but
first we need a mechanism to gain access to the PRCMU
device node. We're going to use a Phandle in this case.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
[Added reg-names="prcmu"; to the prcmu DT node]
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>