ARM: HYP/non-sec: add generic ARMv7 PSCI code

Implement core support for PSCI. As this is generic code, it doesn't
implement anything really useful (all the functions are returning
Not Implemented).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Acked-by: Ian Campbell <ijc@hellion.org.uk>