docs: Added docs/specs/riscv-iommu.rst in MAINTAINERS file.

docs: Added docs/specs/riscv-iommu.rst in MAINTAINERS file.

Added docs/specs/riscv-iommu.rst under `RISC-V TCG CPUs` section in
in MAINTAINERS file since
`scripts/get_maintainer.pl -f docs/specs/riscv-iommu.rst` doesn't list any
maintainers.

Signed-off-by: hemanshu.khilari.foss <hemanshu.khilari.foss@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250323140151.9994-1-hemanshu.khilari.foss@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>