boards: 96b_carbon: provide BT SPI master configuration

On 96Boards Carbon, Bluetooth is provided by a secondary nRF51 chip
connected to SPI_1, so enable this peripheral and its driver when BT
is selected.

Similarly, provide BT_SPI_* configs to integrate with the BT HCI SPI
driver. The files these config values apply to only get built when
CONFIG_BT=y, but this configuration can't be handled in the "if BT"
section in 96b_carbon's Kconfig.defconfig.  This is because BT_SPI is
a choice value, and thus doesn't support a default setting.

Signed-off-by: Marti Bolivar <marti.bolivar@linaro.org>
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Ricardo Salveti <ricardo.salveti@linaro.org>