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>

MAINTAINERS

@@ -319,6 +319,7 @@ L: qemu-riscv@nongnu.org
 S: Supported
 F: configs/targets/riscv*
 F: docs/system/target-riscv.rst
+F: docs/specs/riscv-iommu.rst
 F: target/riscv/
 F: hw/char/riscv_htif.c
 F: hw/riscv/