sw64: simplify pgtable set and populate interfaces

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



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

The pxd_set() and its friends seem ugly and redundant, and this
patch rewirtes these interfaces to make it clear.

Signed-off-by: He Sheng <hesheng@wxiat.com>
Reviewed-by: Cui Wei <cuiwei@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>