samples: Bluetooth: add HCI SPI raw handler sample

Add a sample application that allows a Zephyr-based Bluetooth
controller to interface with an HCI driver via SPI. This sample
implements the same BT SPI protocol already as Zephyr's HCI SPI
driver.

Currently, the sample only supports the legacy SPI API.

Provide a single configuration file, avoiding board-specific
files. Some board-specific configuration information must be provided
via other means:

- CONFIG_BT_CONTROLLER_TO_HOST_SPI_DEV_NAME
- CONFIG_BT_CONTROLLER_TO_HOST_SPI_IRQ_DEV_NAME
- CONFIG_BT_CONTROLLER_TO_HOST_SPI_IRQ_PIN

Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>
Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
Signed-off-by: Michael Scott <michael.scott@linaro.org>