Bluetooth: SPI: introduce CONFIG_BLUETOOTH_SPI_BLUENRG

This new option is used to encapsulate the logic specific to devices
implementing the BlueNRG Bluetooth stack (e.g. X-NUCLEO-IDB05A1).

The current BlueNRG specific logic covers the HCI Reset handling and
the manual control of the SPI Chip Select line (normally not needed since
spi_transceive is also responsible for controlling the SPI CS line).

Change-Id: I5db4addf873eee0af2d957e2181c50aac53ab656
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>