mfd: db8500-prcmu: get base address from resource

We cannot use a global variable stored in <mach/hardware.h> to
find the base address of the PRCMU. The real resource is already
there from the board, so use this to look up the base address
instead.

Currently the patch is kept minimal so as not to interfere with
other work being done on refactoring this driver, but at a later
point the defines using (prcmu_base + 0xnnn) need to be replaced
by pure offset defined for (0xnnn) and the base inlined with the
readl()/writel() and similar codepaths.

Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>