vfio/fsl: Create Kconfig sub-menu

For consistency with pci and platform, push the vfio-fsl-mc option into a
sub-menu.

Reviewed-by: Cédric Le Goater <clg@redhat.com>
Reviewed-by: Eric Auger <eric.auger@redhat.com>
Link: https://lore.kernel.org/r/20230614193948.477036-4-alex.williamson@redhat.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>