Add customizable standalone user port data widths

[craigjb]

Adds data_width parameter in the yml configuration for user ports of types native, AXI, and Wishbone. The parameter is optional, defaulting to previous behavior.

Generation of data width converters is handled by existing functionality in crossbar.py

    "user_ports": {
        "native" : {
            "type": "native",
            "data_width": 32,
        },
        "axi" : {
            "type": "axi",
            "id_width": 3,
            "data_width": 32,
        },
        "wishbone" : {
            "type": "wishbone",
            "data_width": 32,
        }
    },

[enjoy-digital]

Thanks @craigjb, this was not yet integrated since I first wanted to do more tests on this. Have you been able to use it successfully? I would be OK merging it but maybe not using it in the examples for now until this is fully validated.

[enjoy-digital]

This has been merged but we'll wait to do more testing before integrating it in the examples.