tests: drivers: uart: mix_fifo_poll: Add UARTE120 to nrf54h20dk test

Add configuration for UARTE120 fast instance. Use multi-instance capability
of the test to run same test on UARTE120 and slow UARTE peripheral.

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