dm: support iothread

Supply a decidate thread, which can moniter a set of fds with epoll,
when the data is ready, call the corresponding callback.

This iothread will be created automatically with the first successful
call to iothread_add,  and will be destroyed in iothread_deinit if it
was created.

Note, currenlty only support one iothread.

Tracked-On: #7940
Signed-off-by: Conghui <conghui.chen@intel.com>
Acked-by: Wang, Yu1 <yu1.wang@intel.com>

devicemodel/include/iothread.h

@@ -0,0 +1,19 @@
+/* Copyright (C) 2022 Intel Corporation.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *
+ */
+
+#ifndef	_iothread_CTX_H_
+#define	_iothread_CTX_H_
+
+struct iothread_mevent {
+	void (*run)(void *);
+	void *arg;
+};
+int iothread_add(int fd, struct iothread_mevent *aevt);
+int iothread_del(int fd);
+int iothread_init(void);
+void iothread_deinit(void);
+
+#endif