scp: Introduce css_scp_system_off() function

The common implementation of css_scp_sys_shutdown and
css_scp_warm_reset is refactored into a new function,
css_scp_system_off() that allows the desired power state to be
specified.

The css_scp_system_off can be used in the implementation of
SYSTEM_RESET2 for PSCI v1.1.

Change-Id: I161e62354d3d75f969b8436d794335237520a9a4
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
2 files changed