drivers: misc: coresight: nrf_etr: Add support for shell

Extend ETR handler to be able to work as shell backend and multiplex
Coresight STM logging with shell content on the UART used by the
ETR handler. Shell backend works only in the standalone mode.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>