hw/riscv/riscv-iommu: add hpm events mmio write

hw/riscv/riscv-iommu: add hpm events mmio write

To support hpm events mmio writes, done via
riscv_iommu_process_hpmevt_write(), we're also adding the 'hpm-counters'
IOMMU property that are used to determine the amount of counters
available in the IOMMU.

Note that everything we did so far didn't change any IOMMU behavior
because we're still not advertising HPM capability to software. This
will be done in the next patch.

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: <20250224190826.1858473-9-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>