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>

include/devicetree/spi.h

@@ -116,6 +116,46 @@ extern "C" {
  */
 #define DT_SPI_DEV_HAS_CS_GPIOS(spi_dev) DT_SPI_HAS_CS_GPIOS(DT_BUS(spi_dev))
 
+/**
+ * @brief Get a SPI device's chip select devicetree specification
+ *
+ * Example devicetree fragment:
+ *
+ * @code{.devicetree}
+ *     gpio1: gpio@... { ... };
+ *
+ *     gpio2: gpio@... { ... };
+ *
+ *     spi@... {
+ *             compatible = "vnd,spi";
+ *             cs-gpios = <&gpio1 10 GPIO_ACTIVE_LOW>,
+ *                        <&gpio2 20 GPIO_ACTIVE_LOW>;
+ *
+ *             a: spi-dev-a@0 {
+ *                     reg = <0>;
+ *             };
+ *
+ *             b: spi-dev-b@1 {
+ *                     reg = <1>;
+ *             };
+ *     };
+ * @endcode
+ *
+ * Example usage:
+ *
+ * @code{.c}
+ *     DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET(DT_NODELABEL(a)) \
+ *           // { DEVICE_DT_GET(DT_NODELABEL(gpio1)), 10, GPIO_ACTIVE_LOW }
+ *     DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET(DT_NODELABEL(b)) \
+ *           // { DEVICE_DT_GET(DT_NODELABEL(gpio2)), 20, GPIO_ACTIVE_LOW }
+ * @endcode
+ *
+ * @param spi_dev a SPI device node identifier
+ * @return #gpio_dt_spec struct corresponding with spi_dev's chip select
+ */
+#define DT_SPI_DEV_CS_GPIOS_DT_SPEC_GET(spi_dev) \
+	GPIO_DT_SPEC_GET_BY_IDX(DT_BUS(spi_dev), cs_gpios, DT_REG_ADDR(spi_dev))
+
 /**
  * @brief Get a SPI device's chip select GPIO controller's node identifier
  *