ARM: ux500: Remove ATAG booting support for Snowball

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>