sw64: simplify pgtable helpers

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



--------------------------------

This patch:
  - Remove unused macros.
  - Rename some variables and macros to make them shorter.
  - Add pfn_to_virt and virt_to_pfn to make code cleaner.
  - Map vaddr by __va().
  - Redefine mk_pte with pfn_to_pte.
  - Fix some wrong usages, like pgd_offset

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

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