Hi!
Usually VMware updates EVC levels with vcenter updates.
For example Broadwell EVC level appeared in vcenter 6.5, however you could use broadwell CPUs in vsphere 6.0 without any problem. That was only EVC level missing.
So I suppose that Skylake EVC level will be added in something like 6.5U2 or the next major release.
At the moment you can just keep EVC disabled or enable it with Broadwell level and upgrade it as soon as it appears in the next version.