arm64: kernel: add __init marker to PSCI init functions

PSCI init functions must be marked as __init so that they are freed
by the kernel upon boot.

This patch marks the PSCI init functions as such since they need not
be persistent in the kernel address space after the kernel has booted.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>