armv8: Add psci.h from the Linux kernel

This commit adds the psci.h header file from Linux kernel
which contains definitions related to the PSCI interface provided
by firmware

Signed-off-by: Sergey Temerkhanov <s.temerkhanov@gmail.com>
Signed-off-by: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>