tests: drivers: spi: spi_loopback: Cleanup after nRF54LM20 rename

Remove duplicated test configuration after
nrf54l20pdk/nrf54l20/cpuxxx was renamed to
nrf54lm20dk/nrf54lm20a/cpuxxx.

Run test with 16MHz/32MHz bitrate using spi00 instance
(spi2x instances support up to 16MHz/2).

Signed-off-by: Sebastian Głąb <sebastian.glab@nordicsemi.no>