doc: bindings: soft-spi: Remove the usage of deprecated properties

According to Documentation/devicetree/bindings/spi/spi-gpio.yaml
from Linux, the recommended spio-gpio properties are:

sck-gpios, miso-gpios and mosi-gpios.

gpio-sck, gpio-mosi and gpio-miso are considered deprecated.

Update the bindings to suggest the recommeded properties.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>