cputlb: ensure _cmmu helper functions follow the naming standard

cputlb: ensure _cmmu helper functions follow the naming standard

We document this in docs/devel/load-stores.rst so lets follow it. The
32 bit and 64 bit access functions have historically not included the
sign so we leave those as is. We also introduce some signed helpers
which are used for loading immediate values in the translator.

Fixes: 282dffc8
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20191021150910.23216-1-alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>