tests: drivers: uart: mix_fifo_poll: Tweak testcase.yaml

Do not use CONFIG_UART_NRFX_UARTE_LEGACY_SHIM=n because this option
is going to be deprecated and this option does not support all uart
instances (e.g. uart120). Limit testing of configuration with
CONFIG_UART_0_ENHANCED_POLL_OUT=y to legacy platforms. nrf54x
platforms have ENDTX-STOPTX short so this feature is not supported
and needed there.

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