sw64: fix get_cpu_freq() bug

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



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The cpu_desc.frequency used to be unmodified in the presence of
CPU frequency scaling, and get_cpu_freq() always return initial
value. It has to maintain a constant correlation to the target
frequency.

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