[PATCH] spi: dt-bindings: remove unused required property

[Li-hao Kuo <lhjeff911@xxxxxxxxx>]

fix issue
/builds/robherring/linux-dt/Documentation/devicetree/bindings/spi/spi-sunplus-sp7021.example.dt.yaml:
spi@9C002D80: 'clocks-names' is a required property