SPM: Initialize SP args as expected by cm library

In the context management library, cm_setup_context() takes the
information in ep_info to fill the registers x0-x7. This patch replaces
the current code that sets them manually by the correct initialization
code.

Change-Id: Id1fdf4681b154026c2e3af1f9b05b19582b7d16d
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>