mem-ruby: add priorities in SimpleNetwork routing

Configurations can specify a routing priority for message buffers.
This priority is used by SimpleNetwork when checking for messages
in the routers' input ports. Higher priority ports are always checked
first.

JIRA: https://gem5.atlassian.net/browse/GEM5-920

Change-Id: I7e2b35e2cae63086a76def1145f9b4b56220a2ba
Signed-off-by: Tiago Mück <tiago.muck@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41864
Reviewed-by: Meatboy 106 <garbage2collector@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed