cpu_ldst.h: Allow NB_MMU_MODES to be 7

Support guest CPUs which need 7 MMU index values.
Add a comment about what would be required to raise the limit
further (trivial for 8, TCG backend rework for 9 or more).

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