spi: convert CS usage to `gpio_dt_spec`

Convert all CS control logic to be based on the `gpio_dt_spec` member
instead of the standalone `port`, `pin` and `flags` members.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>
Co-authored-by: Jordan Yates <jordan.yates@data61.csiro.au>
Signed-off-by: Bartosz Bilas <bartosz.bilas@hotmail.com>
Co-authored-by: Bartosz Bilas <bartosz.bilas@hotmail.com>