bl2: add bl2_plat_handle_pre_image_load()

bl2: add bl2_plat_handle_pre_image_load()

There are cases where we need to manipulate image information before
the load.  For example, for decompressing data, we cannot load the
compressed images to their final destination.  Instead, we need to
load them to the temporary buffer for the decompressor.

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