python: Make Port roles a more generic concept.

python: Make Port roles a more generic concept.

A recent change got rid of the strict Master => Slave port relationship
which used to be checked in python and instead left the checking up to
C++. One major downside to this approach is that it was no longer
obvious in the configuration what was supposed to be connected to what,
and it still left the arbitrary and misleading MasterPort and SlavePort
types in the Ethernet devices which could now connect with each other
symmetrically but couldn't actually connect to an arbitrary
MasterPort/SlavePort.

This change exposes the base Port and VectorPort types, and makes them
accept a "role" parameter in __init__ which used to be set directly by
their subclasses. This role can be any string, and will be used later
to check for compatiblity and to give a hint as to what can be
connected to what in the SimObject definitions.

To make the checks work with arbitrary compatible pairs, the base Port
type now has a class method called compat() which accepts a pair of
roles which will become mutually compatible, ie any port with the first
role will be allowed to connect to any port with the second role, and
vice versa. To be self compatible, the same role should be passed in
for both parameters.

To maintain compatibility, the MasterPort and SlavePort types are
retained, but now they're nothing special and could have been set up
in any arbitrary SimObject .py file. The same is true for
MasterVectorPort and SlaveVectorPort.

Also, since we can no longer assume that all edges in the dot graph of
the config should start with a port with the MASTER role and end with
a port with the SLAVE role, Ports now track an is_source property which
says whether the arrow head should be surpressed at that end of the
edge representing the connection.

Change-Id: Ifcc6faab05e437ad87cd21f0ba613b09cf21c321
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18168
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>