spi: helper macro for constructing CS gpio_dt_spec

Adds a helper macro, `DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET`, that constructs
a `struct gpio_dt_spec` corresponding with the CS gpio of an spi device.

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>