riscv: perf_event: Make some funciton static

Fixes the following warning detected when running make with W=1,
../arch/riscv/kernel/perf_event.c:150:5: warning: no previous prototype for ‘riscv_map_cache_decode’ [-Wmissing-prototypes]
 int riscv_map_cache_decode(u64 config, unsigned int *type,
     ^~~~~~~~~~~~~~~~~~~~~~
../arch/riscv/kernel/perf_event.c:345:13: warning: no previous prototype for ‘riscv_base_pmu_handle_irq’ [-Wmissing-prototypes]
 irqreturn_t riscv_base_pmu_handle_irq(int irq_num, void *dev)
             ^~~~~~~~~~~~~~~~~~~~~~~~~
../arch/riscv/kernel/perf_event.c:364:6: warning: no previous prototype for ‘release_pmc_hardware’ [-Wmissing-prototypes]
 void release_pmc_hardware(void)
      ^~~~~~~~~~~~~~~~~~~~
../arch/riscv/kernel/perf_event.c:467:12: warning: no previous prototype for ‘init_hw_perf_events’ [-Wmissing-prototypes]
 int __init init_hw_perf_events(void)
            ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>