boards: 96b_carbon: add gpios in bt controller node

Use DT to generate defines for irq and reset gpios in bluetooth
controller node.

Define a bt alias to allow generation of defines with friendly
names.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>
Signed-off-by: Yannis Damigos <giannis.damigos@gmail.com>