hw/riscv/riscv-iommu: instantiate hpm_timer

hw/riscv/riscv-iommu: instantiate hpm_timer

The next HPM related changes requires the HPM overflow timer to be
initialized by the riscv-iommu base emulation.

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: <20250224190826.1858473-6-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>