cpu: Increased the upper bound of cpu issue width

The upper bound was 8, but issue width increased to 12 since Cortex A77.
(https://en.wikichip.org/wiki/arm_holdings/microarchitectures/cortex-a77)
This CL increased the upper bound to 12.

Bug: 175759373
Test: gem5/main/config/example/se.py
Change-Id: I9d084b940628a2bcfa676d386d3d1a82ba9b03f2
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44225
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/o3/impl.hh

@@ -72,7 +72,7 @@ struct O3CPUImpl
 
     enum
     {
-      MaxWidth = 8,
+      MaxWidth = 12,
       MaxThreads = 4
     };
 };