MAINTAINERS: Paul to co-maintain vendor-independent IOMMU code

Having just a single maintainer is not helpful anywhere, and can be
avoided here quite easily, seeing that Paul has been doing quite a bit
of IOMMU work lately.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Julien Grall <julien@xen.org>
Reviewed-by: Paul Durrant <pdurrant@amazon.com>

MAINTAINERS

@@ -323,6 +323,7 @@ F:	xen/arch/x86/cpu/vpmu_intel.c
 
 IOMMU VENDOR INDEPENDENT CODE
 M:	Jan Beulich <jbeulich@suse.com>
+M:	Paul Durrant <pdurrant@amzn.com>
 S:	Supported
 F:	xen/drivers/passthrough/
 X:	xen/drivers/passthrough/amd/