hw/riscv/riscv-iommu.c: check for reserved PTE bits

We need to fault if reserved PTE bits (60:54) are set.

Fixes: 0c54acb8243d ("hw/riscv: add RISC-V IOMMU base emulation")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/3554
Signed-off-by: Daniel Henrique Barboza <daniel.barboza@oss.qualcomm.com>
Reviewed-by: Nutty Liu <nutty.liu@hotmail.com>
Reviewed-by: Chao Liu <chao.liu.zevorn@gmail.com>
Message-ID: <20260701121111.537654-2-daniel.barboza@oss.qualcomm.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>