sim: Allow passing a user-defined L2XBar to addTwoLevelCacheHierarchy().
sim: Allow passing a user-defined L2XBar to addTwoLevelCacheHierarchy().

Before this CL, the addTwoLevelCacheHierarchy() function uses the
default L2XBar class as the interconnect between CPU L1 caches and
L2. This CL allows passing a user-defined bus to overwrite the
default L2XBar by adding an optional argument to the function.

Change-Id: I917657272fd4924ee0bed882a226851afba26847
Reviewed-on: https://gem5-review.googlesource.com/7364
Reviewed-by: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Jason Lowe-Power <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>

TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5