spi: dt-bindings: remove unused required property

fix issue
/builds/robherring/linux-dt/Documentation/devicetree/bindings/spi/spi-sunplus-sp7021.example.dt.yaml:
spi@9C002D80: 'clocks-names' is a required property
From schema: /builds/robherring/linux-dt/Documentation/devicetree/bindings/spi/spi-sunplus-sp7021.yaml
delete unused required(clock-name)

Fixes: 3b8ab4da ("spi: Fix test error for sp7021")

Reported-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Li-hao Kuo <lhjeff911@gmail.com>
Link: https://lore.kernel.org/r/097bbc8b703b17e8fb3e3f6f6d2f97fe668bd5c5.1645770648.git.lhjeff911@gmail.com


Signed-off-by: Mark Brown <broonie@kernel.org>