MAINTAINERS: Add myself as a reviewer for XIVE

MAINTAINERS: Add myself as a reviewer for XIVE

Proposing myself as a reviewer for XIVE on PPC.

I have been looking at XIVE in context of KVM internally at IBM for some time
in addition to testing a few XIVE upstream patches; and I'll be closely looking
at XIVE going forward.

Signed-off-by: Gautam Menghani <gautam@linux.ibm.com>
Signed-off-by: Harsh Prateek Bora <harshpb@linux.ibm.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Link: https://lore.kernel.org/qemu-devel/20250724100623.3071131-6-harshpb@linux.ibm.com
Signed-off-by: Cédric Le Goater <clg@redhat.com>