MAINTAINERS: add Xen Domain-0 snippet to Xen Platform

The snippet for Zephyr as Xen Domain-0 was missing in the
Xen Platform section of MAINTAINERS.yml.

Add the missing entry to ensure that future pull requests
for this component are correctly tracked and assigned.

Signed-off-by: Dmytro Firsov <dmytro_firsov@epam.com>