target/riscv: Add support for hpmcounters/hpmevents

target/riscv: Add support for hpmcounters/hpmevents

With SBI PMU extension, user can use any of the available hpmcounters to
track any perf events based on the value written to mhpmevent csr.
Add read/write functionality for these csrs.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220620231603.2547260-7-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>