This patch adds a couple of missing forward declarations in plat_arm.h
so that all types it references are known within this header file,
without relying on previous header inclusions. This concerns the
meminfo and bl31_params structures, which are defined in bl_common.h.
Other external types referenced from plat_arm.h (e.g. mmap_region_t)
get declared through header files included by arm_plat.h so they
don't need forward declarations.

Change-Id: I471d5aa487919aff3fa979fc65e053f4f5b0ef32