Revert "arm64: hibernate: Refuse to hibernate if the boot cpu is offline"

Now that we use the MPIDR to resume on the same CPU that we hibernated on,
we no longer need to refuse to hibernate if the boot cpu is offline. (Which
we can't possibly know if kexec causes logical CPUs to be renumbered).

This reverts commit 1fe492ce.

Signed-off-by: James Morse <james.morse@arm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>