tests: drivers: i2s_api: Allow testing on nRF52840 DK and nRF5340 DK

Provide configurations for the nRF52840 DK and nRF5340 DK boards.
Adjust the test to cover specifics of I2S peripherals on nRF SoCs
(need of starting RX and TX simultaneously, lack of internal loopback).

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>