SPMD: extract SPMC DTB header size from SPMD

SPMD: extract SPMC DTB header size from SPMD

Currently BL2 passes TOS_FW_CONFIG address and size through registers to
BL31. This corresponds to SPMC manifest load address and size. The SPMC
manifest is mapped in BL31 by dynamic mapping. This patch removes BL2
changes from generic code (which were enclosed by SPD=spmd) and retrieves
SPMC manifest size directly from within SPMD. The SPMC manifest load
address is still passed through a register by generic code.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I35c5abd95c616ae25677302f0b1d0c45c51c042f