cpu: Increased the upper bound of cpu issue width
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 <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5,gem5/gem5