Add a way to specify I/O port attributes at the time of buffer instantiation

[whitequark]

Right now this is only possible when creating a Resource; however, there are many attributes (e.g. SLEW_RATE) which should be configured by the I/O component rather than the toplevel or the board file.

This could be done via the platform, e.g. using platform.add_port_attribute(port, "SLEW_RATE", "FAST") where port could be a SingleEndedPort or DifferentialPort with IOPorts that the platform recognizes.