target/arm: Introduce arm_mmu_idx

target/arm: Introduce arm_mmu_idx

The pattern

  ARMMMUIdx mmu_idx = core_to_arm_mmu_idx(env, cpu_mmu_index(env, false));

is computing the full ARMMMUIdx, stripping off the ARM bits,
and then putting them back.

Avoid the extra two steps with the appropriate helper function.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20190108223129.5570-17-richard.henderson@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>