soc: nordic: nrf54h: cpuapp: Don't use serial shell when ETR is used

ETR handler (for Coresight STM logging) is using console UART and
can act as the shell backend. When that happens default serial shell
backend shall be disabled (and it is by default enabled if there is
a zephyr,console chosen set.

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