target/arm: Use cpu_*_mmu instead of helper_*_mmu

target/arm: Use cpu_*_mmu instead of helper_*_mmu

The helper_*_mmu functions were the only thing available
when this code was written.  This could have been adjusted
when we added cpu_*_mmuidx_ra, but now we can most easily
use the newest set of interfaces.

Cc: qemu-arm@nongnu.org
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>