dts: bindings: xen: add xen,xen.yaml file

Add yaml file for 'xen,xen', because without it an appropriate
'CONFIG_DT_HAS_XEN_XEN_ENABLED' isn't generated.

It will be used for checking Xen support on current setup, instead of
checking if it is BOARD/SOC "xenvm" (which is not correct for Domain-0
configurations).

Remove xen,xen-4.15.yaml at all, because it isn't necessary to have
yaml for some specific Xen version.

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