sw64: map logical address with __va()

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I5G5YB



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Although kmem and IO space are accessed with direct mapping on sw64,
it's not a standard method to or PAGE_OFFSET and access it directly
everywhere. This patch maps PA to VA with __va(). Besides, we change
type of some variables to pointer to make the code clearer.

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

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