Minor refactor of BL2 image load v2

Minor refactor of BL2 image load v2

Previously, get_next_bl_params_from_mem_params_desc() populated arg0
in the EL3 runtime entrypoint with a bl_params_t pointer. This is the
responsibility of the generic LOAD_IMAGE_V2 framework instead of the
descriptor-based image loading utility functions. Therefore this patch
moves that code to bl2_load_images().

Also, this patch moves the code that flushes the bl_params structure to
flush_bl_params_desc(), together with the other descriptor-based image
loading flushing code.

Change-Id: I4541e3f50e3878dde7cf89e9e8f31fe0b173fb9d
Signed-off-by: Dan Handley <dan.handley@arm.com>