Documentation: devicetree: spi: fix wrong spi-bus documentation

This patch adds missing commas to the spi-bus documentation of the
cs-gpio lines.

The device tree compiler fails if chip select lines are not
comma-separated. Fix the erroneous documentation by adding missing
commas.

Signed-off-by: Guenther Wutz <info@gunibert.de>
Signed-off-by: Ralf Ramsauer <ralf@ramses-pyramidenbau.de>
Signed-off-by: Rob Herring <robh@kernel.org>