sw64: add DVFS support for different boards

Sunway inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I56QDM



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This patch add two supports:
    * Initialize frequency table according to frequency of external
      crystal.
    * Set the initial frequency as the higest one to avoid exceeding
      the limit on some boards.

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