ath79: remove num-chipselects from SPI controller

ath79: remove num-chipselects from SPI controller

The num-chipselects property is only valid for bit-banged spi-gpio [1].

For the generic SPI controller bindings, num-cs would be the correct
property [2].

Since num-cs is not required on ath79 either, just drop the lines.

[1] https://www.kernel.org/doc/Documentation/devicetree/bindings/spi/spi-gpio.yaml
[2] https://www.kernel.org/doc/Documentation/devicetree/bindings/spi/spi-controller.yaml

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>