gpio: sx1509b: Add device tree support for GPIO generation

Add the missing bits to the yaml & dts to enable GPIO pin generation
based on device tree.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>

boards/arm/nrf52_pca20020/nrf52_pca20020.dts

@@ -51,6 +51,8 @@
 		compatible = "semtech,sx1509b";
 		reg = <0x3e>;
 		label = "GPIO_P0";
+		gpio-controller;
+		#gpio-cells = <2>;
 	};
 
 	lps22hb_press: lps22hb_press@5c {

dts/bindings/gpio/semtech,sx1509b-gpio.yaml

@@ -17,4 +17,16 @@ inherits:
 properties:
     compatible:
       constraint: "semtech,sx1509b"
+
+    label:
+      type: string
+      category: required
+      description: Human readable string describing the device (used by Zephyr for API name)
+      generation: define
+
+cell_string: GPIO
+
+"#cells":
+  - pin
+  - flags
 ...