ipq806x: dts: fix SPI chip select GPIO polarity

ipq806x: dts: fix SPI chip select GPIO polarity

The SPI chip select GPIO polarity is active low by default. We must
use "spi-cs-high" dts property to toggle the polarity. The polarity
on "cs-gpios" won't take effect at all[1]. Fix these incorrect GPIO
polarities to silence the kernel warnings.

[1] Refer to Linux/Documentation/devicetree/bindings/spi/spi-controller.yaml
```
      device node     | cs-gpio       | CS pin state active | Note
      ================+===============+=====================+=====
      spi-cs-high     | -             | H                   |
      -               | -             | L                   |
      spi-cs-high     | ACTIVE_HIGH   | H                   |
      -               | ACTIVE_HIGH   | L                   | 1
      spi-cs-high     | ACTIVE_LOW    | H                   | 2
      -               | ACTIVE_LOW    | L                   |

      Notes:
      1) Should print a warning about polarity inversion.
         Here it would be wise to avoid and define the gpio as
         ACTIVE_LOW.
      2) Should print a warning about polarity inversion
         because ACTIVE_LOW is overridden by spi-cs-high.
         Should be generally avoided and be replaced by
         spi-cs-high + ACTIVE_HIGH.
```

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19845
Signed-off-by: Hauke Mehrtens <[email protected]>