xen/efi: make Xen PV EFI clock optional

The present implementation is only for x86. Other architectures need
adjustments for querying presence of EFI.

Xen's EFI support is also quite troublesome on non-x86. This is being
slowly remedied, but until in better shape the EFI clock functionality
should be disabled.

Reviewed by: royger
Differential Revision: https://reviews.freebsd.org/D31065