dt-bindings: fpga: xilinx-slave-serial: add optional INIT_B GPIO

The INIT_B is used by the 6 and 7 series to report the programming status,
providing more control and information about programming errors.
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Moritz Fischer <mdf@kernel.org>

Documentation/devicetree/bindings/fpga/xilinx-slave-serial.txt

@@ -16,6 +16,10 @@ Required properties:
 - prog_b-gpios: config pin (referred to as PROGRAM_B in the manual)
 - done-gpios: config status pin (referred to as DONE in the manual)
 
+Optional properties:
+- init-b-gpios: initialization status and configuration error pin
+                (referred to as INIT_B in the manual)
+
 Example for full FPGA configuration:
 
 	fpga-region0 {
@@ -40,7 +44,8 @@ Example for full FPGA configuration:
 			spi-max-frequency = <60000000>;
 			spi-cpha;
 			reg = <0>;
-			done-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
 			prog_b-gpios = <&gpio0 29 GPIO_ACTIVE_LOW>;
+			init-b-gpios = <&gpio0 28 GPIO_ACTIVE_LOW>;
+			done-gpios = <&gpio0 9 GPIO_ACTIVE_HIGH>;
 		};
 	};