Add device model to PADD

[yubiuser]

• What does this PR aim to accomplish?:

Adds the device model to PADD output. Fixes #228

• How does this PR accomplish the above?:

Reading from /sys/firmware/devicetree/base/model

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[yubiuser]

I only added this for regular, slim and bigger sizes. If reviewers think it should be part of smaller sizes it can be added (though not for the smallest)