add support for rs485 mode and enable dtr pin

[antonello-contini-one]

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[bugadani]

Given that RS485 is half-duplex, and our UART driver is full-duplex, I'm not convinced this is how we want to support RS485. Please mark all public APIs you are introducing with #[instability::unstable] for now so that we can revisit.

[MabezDev]

Closing this for now, please feel free to open a new PR with the review comments resolved.