This will allow us to trace 32k-long memory accesses (although our
maximum is something like 256 bytes at the moment).

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Emilio G. Cota <cota@braap.org>
[AJB: expanded to 3->4 bits]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>