samples: drivers: spi_bitbang: Enable sample on nrf54* targets

Add overlays required to run the sample on
- nrf54h20dk/nrf54h20/cpuapp,
- nrf54l15dk/nrf54l15/cpuapp,
- nrf54lm20dk/nrf54lm20a/cpuapp.

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