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

drivers/vfio/fsl-mc/Kconfig

+menu "VFIO support for FSL_MC bus devices"
+	depends on FSL_MC_BUS
+
 config VFIO_FSL_MC
 	tristate "VFIO support for QorIQ DPAA2 fsl-mc bus devices"
-	depends on FSL_MC_BUS
 	select EVENTFD
 	help
 	  Driver to enable support for the VFIO QorIQ DPAA2 fsl-mc
@@ -8,3 +10,5 @@ config VFIO_FSL_MC
 	  fsl-mc bus devices using the VFIO framework.
 
 	  If you don't know what to do here, say N.
+
+endmenu