riscv: perf: RISCV_BASE_PMU should be independent

Selecting PERF_EVENTS without selecting RISCV_BASE_PMU results in a build
error.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
[Palmer: commit text]
Fixes: 178e9fc4

("perf: riscv: preliminary RISC-V support")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>