Bluetooth: hci_bcm: enable IRQ capability from devicetree

Add support for getting IRQ directly from DT instead of relying on
converting a GPIO to IRQ. This is needed for platforms with GPIO
controllers that that do not support gpiod_to_irq().

Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Guillaume La Roque <glaroque@baylibre.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>