target/riscv: pmu: Rename the counters extension to pmu

target/riscv: pmu: Rename the counters extension to pmu

The PMU counters are supported via cpu config "Counters" which doesn't
indicate the correct purpose of those counters.

Rename the config property to pmu to indicate that these counters
are performance monitoring counters. This aligns with cpu options for
ARM architecture as well.

Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220620231603.2547260-4-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>