spi: soft_spi: Support the recommended soft spi 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.

Currently, U-Boot only supports the deprecated properties.

Allow the soft_spi driver to support both the new and old properties.

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