MAINTAINERS: Mark AMD-Vi emulation as orphan

MAINTAINERS: Mark AMD-Vi emulation as orphan

hw/i386/amd_iommu.c seems unmaintained:
After commit 1c7955c450 ("x86-iommu: introduce parent class",
2016-07-14), almost no feature added, 2 bug fixes, other changes
are generic tree-wide API cleanups.

Cc: Roman Kapl <rka@sysgo.com>
Cc: Wei Huang <wei.huang2@amd.com>
Cc: Brijesh Singh <brijesh.singh@amd.com>
Cc: David Kiarie <davidkiarie4@gmail.com>
Cc: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230313153031.86107-2-philmd@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>