allwinner: Fix non-default PRELOADED_BL33_BASE

allwinner: Fix non-default PRELOADED_BL33_BASE

While the Allwinner platform code nominally supported a custom
PRELOADED_BL33_BASE, some references to the BL33 load address used
another constant: PLAT_SUNXI_NS_IMAGE_OFFSET. To allow the DTB search
code to work if a U-Boot BL33 is loaded to a custom address,
consistently use PRELOADED_BL33_BASE. And to avoid this confusion in
the future, remove the other constant.

Signed-off-by: Samuel Holland <samuel@sholland.org>
Change-Id: Ie6b97ae1fdec95d784676aef39200bef161471b0