ARMv8: Add basic PSCI framework

This patch introduces a generic ARMv8 PSCI framework, with all functions
returning a dummy ARM_PSCI_RET_NI (Not Implemented), then it is up to each
platform to implement their own functions based on this framework.

Signed-off-by: Hongbo Zhang <hongbo.zhang@nxp.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: York Sun <york.sun@nxp.com>