boards nrf52_bsim: Provide default UART configuration

Provide a good enough UART configuration, but do not
select it as backend by default, let apps do that.

The UART and its driver are very heavy compared to
other backends it may replace, so let's avoid
enabling it by default.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>