Add a new MMU mode that includes the current virt mode.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 4b301bc0ea36da962fc1605371b65019ac3073df.1604464950.git.alistair.francis@wdc.com