feat(spmd): fail safe if SPM fails to initialize

The spmd_setup function is made fail safe in that a failure in the
SPMC manifest parsing, SPMD or SPMC initialization returns a success
code to the standard services initialization routine (std_svc_setup).
This permits continuing the boot process and initialize services
beyond the SPMD to succeed for the system to operate in the normal
world. It operates in a degraded mode for the secure world.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: Ida0ac91c17925279a79f112d190f9ad038f518e7