samples/bluetooth/hci_spi: Add configuration for nrf51_pca10028

This sample has nrf51_pca10028 on its platform_whitelist but lacks
configuration and overlay files that would make it possible to build
this sample for that board. This commit provides such files.

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