drivers: spi: mark old spi_cs_control fields deprecated

Since gpio_dt_spec is finally available within spi_cs_control
struct, mark the old fields deprecated in order to use
gpio_dt_spec struct.

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>