cpu_ldst.h: Don't define helpers if MMU_MODE*_SUFFIX not defined

Not all targets define a full set of suffix strings for the
NB_MMU_MODES that they have. In this situation, don't define any
helper functions for that mode, rather than defining helper functions
with no suffix at all. The MMU mode is still functional; it is merely
not directly accessible via cpu_ld*_MODE from target helper functions.

Also add an "NB_MMU_MODES >= 2" check to the definition of the mode 1
helpers -- some targets only define one MMU mode.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
Message-id: 1421432008-6786-1-git-send-email-peter.maydell@linaro.org
