arm64: spin-table: add missing of_node_put()

Since of_get_cpu_node() increments refcount, the node should be put.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>