LoongArch: Refine the init_hw_perf_events() function

LoongArch: Refine the init_hw_perf_events() function

(1) Use the existing CPUCFG6_PMNUM_SHIFT macro definition instead of
the magic value 4 to get the PMU number.

(2) Detect the value of PMU bits via CPUCFG instruction according to
the ISA manual instead of hard-coded as 64, because the value may be
different for various micro-architectures.

Link: https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#_cpucfg
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>