sw64: clean up unused pci iounmap operation

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56OLG



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For sw64, IO address is mapped by direct mapping without pages.
It doesn't have to do anything to unmap PCI MMIO. The result is
that __is_mmio helper can be removed.

Signed-off-by: He Sheng <hesheng@wxiat.com>

Signed-off-by: Gu Zitao <guzitao@wxiat.com>