lib: Add a fifo implementation.

Implement a fifo to accomodate outstanding IPIs for a specific hart
at the same time.

Signed-off-by: Atish Patra <atish.patra@wdc.com>

include/sbi/sbi_fifo.h

@@ -0,0 +1,32 @@
+/*
+ * SPDX-License-Identifier: BSD-2-Clause
+ *
+ * Copyright (c) 2019 Western Digital Corporation or its affiliates.
+ *
+ * Authors:
+ *   Atish Patra<atish.patra@wdc.com>
+ *
+ */
+
+#ifndef __SBI_FIFO_H__
+#define __SBI_FIFO_H__
+
+#include <sbi/riscv_locks.h>
+#include <sbi/sbi_types.h>
+
+struct sbi_fifo {
+	int head;
+	int tail;
+	spinlock_t qlock;
+	unsigned long entrysize;
+	unsigned long num_entries;
+	void *queue;
+};
+
+int sbi_fifo_dequeue(struct sbi_fifo *fifo, void *data);
+int sbi_fifo_enqueue(struct sbi_fifo *fifo, void *data);
+void sbi_fifo_init(struct sbi_fifo *fifo, unsigned long entries,
+		   unsigned long entrysize);
+bool sbi_fifo_is_empty(struct sbi_fifo *fifo);
+bool sbi_fifo_is_full(struct sbi_fifo *fifo);
+#endif