ARM/ARM64: KVM: Add common header for PSCI related defines
We need a common place to share PSCI related defines among ARM kernel,
ARM64 kernel, KVM ARM/ARM64 PSCI emulation, and user space.
We introduce uapi/linux/psci.h for this purpose. This newly added
header will be first used by KVM ARM/ARM64 in-kernel PSCI emulation
and user space (i.e. QEMU or KVMTOOL).