spi: mediatek: remove unrequired description

cs-gpios isn't required with patch "spi: mediatek: single
device does not require cs_gpios", so modify the description.

Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>