dt-bindings: net: realtek-bluetooth: Add RTL8821CS

Add compatible string for RTL8821CS for existing Realtek Bluetooth
driver.

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Alistair Francis <alistair@alistair23.me>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>