The PRELOADED_BL33_BASE build option allows to preload a BL33 and bypass its
loading by BL2. In ARM standard platforms, the conditional behaviour of
PRELOADED_BL33_BASE is moved within the implementation of
`plat_get_ns_image_entrypoint()` so that all callers may benefit from this
feature.

Change-Id: Iea060e204ec72f8081087837854535c4e320da4e