mem-ruby: Fix a bug in MessageBuffer randomization

mem-ruby: Fix a bug in MessageBuffer randomization

In the previous implementation, messages are randomly inserted with
delays only if both RubySystem and MessageBuffer randomization flags
are set true.  However, to find race conditions and cover more slicc
transitions, ruby random testers rely on setting RubySystem flag to
turn on randomization on all message buffers.
As a fix, this patch enables a message buffer to have randomization
when either RubySystem or its own flag is set.

Change-Id: I1e076908ff07e5846ebad4f4fc1c8f28d40bbfd4
Reviewed-on: https://gem5-review.googlesource.com/12784
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>