tests: drivers: i2s_speed: Allow testing on nRF52840 DK and nRF5340 DK

This is a follow-up to commit 954dfa75.

Apply adjustments made in the i2s_api test to the i2s_speed one so that
it can also be executed successfully on the nRF DK boards.

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