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

(cherry picked from commit 51adf913e8815bdc928cb72dd378c596b8b15336)