arm64: dts: imx8m: Fix the SPI chipselect polarity

The conversion of the spi-imx driver to use GPIO descriptors
in commit 8cdcd8ae ("spi: imx/fsl-lpspi: Convert to GPIO descriptors")
helped to detect the following SPI chipselect polarity mismatch on an
imx6q-sabresd for example:

[    4.854337] m25p80@0 enforce active low on chipselect handle

Prior to the above commit, the chipselect polarity passed via cs-gpios
property was ignored and considered active-low.

The reason for such mismatch is clearly explained in the comments inside
drivers/gpio/gpiolib-of.c:

 * SPI children have active low chip selects
 * by default. This can be specified negatively
 * by just omitting "spi-cs-high" in the
 * device node, or actively by tagging on
 * GPIO_ACTIVE_LOW as flag in the device
 * tree. If the line is simultaneously
 * tagged as active low in the device tree
 * and has the "spi-cs-high" set, we get a
 * conflict and the "spi-cs-high" flag will
 * take precedence.

To properly represent the SPI chipselect polarity, change it to active-low
when the "spi-cs-high" property is absent.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>