arm64: disable the physical timer for now if HCR_EL2.E2H is set

[kevans]

Summary

On some hardware, we can't clear HCR_EL2.E2H so accesses to the physical
timer hopelessly trap to EL2. Stash off the value of HCR_EL2 and use it
in has_hyp() to avoid this.

[andrew]

Will also work with bhyve where we don't currently work with HCR_E2H on.