drivers: serial: nrfx_uarte: Improve RX FIFO flush workaround

Add configurable magic byte instead of fixed 0xAA. In some cases, it
is known that certain bytes are less likely in the transmission and
picking specific magic byte may reduce probability of failure of
detection of the correct amount of flushed data.

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