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

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 7c0e8d506b745e012b6e7341adbbab4ae51dcc80.

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