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/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 */