mem-ruby: Network can use custom data msg size

mem-ruby: Network can use custom data msg size

The size for network data messages can be set using a configuration
parameter. This is necessary so line transfers may be split in multiple
messages at the protocol level.

Change-Id: I86a272de597b04a898071db412b921cbe1651ef0
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/31256
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Srikant Bharadwaj <srikant.bharadwaj@amd.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>