sw64: increase the minimun frequency of DVFS

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I73OKY



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It will make the system run smoothly when DVFS is working.

Signed-off-by: Tang Jinyang <tangjinyang@wxiat.com>
Reviewed-by: He Sheng <hesheng@wxiat.com>
Signed-off-by: Gu Zitao <guzitao@wxiat.com>