spi: spi-gpio: Add dt support for a single device with no chip select

In order to describe a single slave device that has no chip select line
the 'num-chipselects' property has to be <0> and the 'cs-gpios' property
doesn't need to be set.

Signed-off-by: Torsten Fleischer <torfl6749@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>