hw/riscv/riscv-iommu.c: add RISCV_IOMMU_CAP_HPM cap

hw/riscv/riscv-iommu.c: add RISCV_IOMMU_CAP_HPM cap

Now that we have every piece in place we can advertise CAP_HTM to
software, allowing any HPM aware driver to make use of the counters.

HPM is enabled/disabled via the 'hpm-counters' attribute. Default value
is 31, max value is also 31. Setting it to zero will disable HPM
support.

Signed-off-by: Tomasz Jeznach <tjeznach@rivosinc.com>
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250224190826.1858473-10-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>