spi: use `gpio_dt_spec` in `struct spi_cs_control`

Use the standard `struct gpio_dt_spec` type in the chip select control.
The anonymous struct and union allows previous instantiations to
continue working while letting functions operate on the new type.

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>