dt-bindings: net: bluetooth: Add support for Amlogic Bluetooth

Add binding document for Amlogic Bluetooth chipsets attached over UART.

Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Yang Li <yang.li@amlogic.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>