target/riscv: Use existing PMU counter mask in FDT generation

During the FDT generation use the existing mask containing the enabled
counters rather then generating a new one. Using the existing mask will
support the use of discontinuous counters.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Message-ID: <20231031154000.18134-4-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>