Add device interface specification

[bruno-f-cruz]

This is a first stab at describing the device interface, device.yml specification.
It provides a high-level description on how the document is organized, validated and used by downstream tooling. I also provides a few examples.

I think we could add more detail in the following points:

• converter
• interfaceType
• The way that @glopesdev ended up resolving the heterogeneous registers Heterogenous register types in yaml schema #40

[bruno-f-cruz]

We should also address #214
One idea is that scalar registers should NEVER specify length. If length is specied (even if 0) the register interface must always return a array type