arm64: fix NULL dereference in have_cpu_die()

Commit 5c492c3f ("arm64: smp: Add function to determine if cpus are
stuck in the kernel") added a helper function to determine if die() is
supported in cpu_ops. This function assumes a cpu will have a valid
cpu_ops entry, but that may not be the case for cpu0 is spin-table or
parking protocol is used to boot secondary cpus. In that case, there
is a NULL dereference if have_cpu_die() is called by cpu0. So add a
check for a valid cpu_ops before dereferencing it.

Fixes: 5c492c3f ("arm64: smp: Add function to determine if cpus are stuck in the kernel")
Signed-off-by: Mark Salter <msalter@redhat.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>