arm64: disable the EL2 MMU before dropping to EL1

An earlier stage may have set HCR_EL2.E2H, the clearing of which may
break address translation. We don't need the EL2 MMU at this point, so
we can avoid re-enabling it for now and just drop to EL1 as usual.

Suggested by: andrew
Reviewed by: andrew
Differential Revision: https://reviews.freebsd.org/D34644