serial_bt: support the dialog DA14580 chip (seen in TC66C)

serial_bt: support the dialog DA14580 chip (seen in TC66C)

Add support for dialog DA14580 chips, accept conn=bt/dialog/$MAC specs.
These BLE chips were seen in RDTech TC66C energy meters. They appear to
send notifications without the necessity to write CCCD values.

It's yet to get determined whether it's the chip or the device firmware
which requires MTU exchange. The default value in the 400 range (near
the absolute maximum MTU for BLE communication) is rather arbitrary a
choice, needs reconsideration in the future. Users always can override
as desired.