docs/specs/riscv-iommu: Fixed broken link to external risv iommu document

docs/specs/riscv-iommu: Fixed broken link to external risv iommu document

The links to riscv iommu specification document are incorrect. This patch
updates all the said link to point to correct location.

Cc: qemu-stable@nongnu.org
Cc: qemu-riscv@nongnu.org
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2808
Signed-off-by: hemanshu.khilari.foss <hemanshu.khilari.foss@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250323063404.13206-1-hemanshu.khilari.foss@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>