boards: posix: nrf_bsim: Use legacy UART shim

New UART shim must be adapted to work in the simulated environment.
Use legacy version until it is fixed.

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

boards/posix/nrf_bsim/Kconfig.defconfig

@@ -91,4 +91,7 @@ config POSIX_ARCH_CONSOLE
 
 endif # CONSOLE
 
+config UART_NRFX_UARTE_LEGACY_SHIM
+	default y
+
 endif # SOC_SERIES_BSIM_NRFXX