There is no such property in the SPI controller binding documentation.
Also Linux driver doesn't look for it.

This fixes:
arch/arm/boot/dts/bcm4708-asus-rt-ac56u.dtb: spi@18029200: Unevaluated properties are not allowed ('clock-names' was unexpected)
        From schema: Documentation/devicetree/bindings/spi/brcm,spi-bcm-qspi.yaml
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Link: https://lore.kernel.org/r/20230503122830.3200-1-zajec5@gmail.comSigned-off-by: Florian Fainelli <f.fainelli@gmail.com>