Turning the MMU on is a popular sport in the arm64 kernel, and
we do it more than once, or even twice. As we are about to add
even more, let's turn it into a macro.

No expected functional change.
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Acked-by: NCatalin Marinas <catalin.marinas@arm.com>
Acked-by: NDavid Brazdil <dbrazdil@google.com>
Link: https://lore.kernel.org/r/20210208095732.3267263-4-maz@kernel.orgSigned-off-by: NWill Deacon <will@kernel.org>