target-arm: Use mmu_idx in get_phys_addr()

target-arm: Use mmu_idx in get_phys_addr()

Now we have the mmu_idx in get_phys_addr(), use it correctly to
determine the behaviour of virtual to physical address translations,
rather than using just an is_user flag and the current CPU state.

Some TODO comments have been added to indicate where changes will
need to be made to add EL2 and 64-bit EL3 support.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Greg Bellows <greg.bellows@linaro.org>