ARM: ux500: Purge DB8500 PRCMU registration when not booting with DT

It's time to remove all ATAG support from ux500 and rely solely on
Device Tree booting. This patch is part of that endeavour.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>