ARM: dts: gemini: Set DIR-685 SPI CS as active low

The SPI to the display on the DIR-685 is active low, we were
just saved by the SPI library enforcing active low on everything
before, so set it as active low to avoid ambiguity.

Link: https://lore.kernel.org/r/20190715202101.16060-1-linus.walleij@linaro.org
Cc: stable@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Olof Johansson <olof@lixom.net>