target/riscv: Fix the hpmevent mask

target/riscv: Fix the hpmevent mask

As per the latest privilege specification v1.13[1], the sscofpmf
only reserves first 8 bits of hpmeventX. Update the corresponding
masks accordingly.

[1]https://github.com/riscv/riscv-isa-manual/issues/1578

Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250206-pmu_minor_fixes-v2-1-1bb0f4aeb8b4@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>