The virtual timer is optional on ARM64. Properly handle that condition. [1]
In case we do not have an interrupt assignment for the virtual timer,
force the physical timer.
Also skip resource allocation for any timer we do not have an interrupt
assignment for.

In collaboration with: andrew
Submitted by: br ([1] from his gem5 arm64 work)
Sponsored by: DARPA/AFRL
Reviewed by: andrew
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D6203