ARM: ux500: Enable PRCMU Timer 4 (clocksource) for Device Tree

In dbx500 based devices the PRCMU Timer 4 is used as a clocksource
and sched_clock. Here we fetch all necessary addressing information
required for correct PRCMU initialisation from the Device Tree
instead of using hard-coded values.

CC: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>