irq: Tidy up of-platdata irq support

This function is available but not exported. More generally it does not
really work as intended.

Reimplement it and add a sandbox test too.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/dts/sandbox.dtsi

@@ -124,6 +124,19 @@
 		#sound-dai-cells = <1>;
 	};
 
+	irq_sandbox: irq-sbox {
+		u-boot,dm-spl;
+		compatible = "sandbox,irq";
+		interrupt-controller;
+		#interrupt-cells = <2>;
+	};
+
+	irq-test {
+		u-boot,dm-spl;
+		compatible = "sandbox,irq-test";
+		interrupts-extended = <&irq_sandbox 3 0>;
+	};
+
 	lcd {
 		u-boot,dm-pre-proper;
 		compatible = "sandbox,lcd-sdl";

arch/sandbox/include/asm/irq.h

@@ -0,0 +1,20 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright 2021 Google LLC
+ */
+
+#ifndef __SANDBOX_IRQ_H
+#define __SANDBOX_IRQ_H
+
+/**
+ * struct sandbox_irq_priv - private data for this driver
+ *
+ * @count: Counts the number calls to the read_and_clear() method
+ * @pending: true if an interrupt is pending, else false
+ */
+struct sandbox_irq_priv {
+	int count;
+	bool pending;
+};
+
+#endif /* __SANDBOX_IRQ_H */