uniphier: use more mmap_add_dynamic_region() for loading images

Currently, uniphier_bl2_mmap hard-codes the memory region needed for
loading other images.

Towards the goal of making this really position-independent, call
mmap_add_dynamic_region() before that region gets accessed.

Change-Id: Ieb505b91ccf2483e5f1a280accda564b33f19f11
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>