tests: drivers: uart: uart_mix_fifo_poll: Enable device PM

uart120 requires device runtime to be enable. Enable it for nrf54h20dk.

When device runtime PM is used for interrupt driven and polling API
then UART device is initially suspended. It means that RX is disabled.
In order to enable RX device must be explicitly resumed using PM API.

Test is enabling UART RX (uart_rx_enable) from counter callback
(interrupt handler context). For fast instance on nrf54h20dk (uart120)
it is not allowed because PM resume can only be called from the thread
context. Because of that, test is skipped for uart120 and asynchronous
API.

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