dt-bindings: bluetooth: marvell: add 88W8997

Update the documentation with the device tree binding for the Marvell
88W8997 bluetooth device.

Signed-off-by: Stefan Eichenberger <stefan.eichenberger@toradex.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Francesco Dolcini <francesco.dolcini@toradex.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>