ARM: dts: imx6ul-14x14-evk: Fix SPI chipselect polarity

The GPIO expander SPI chipselect is active low. Mark it as such to avoid
the following warning:

[    6.839213] gpio@0 enforce active low on chipselect handle

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>