include: introduce include/linux/io.h as a wrapper of asm/io.h

In the latest Linux coding style, <linux/io.h> should be included
rather than <asm/io.h>.  To follow this standard also in U-Boot,
add include/linux/io.h.  Currently, it just includes <asm/io.h>.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

include/linux/io.h

+/*
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef _LINUX_IO_H
+#define _LINUX_IO_H
+
+#include <asm/io.h>
+
+#endif /* _LINUX_IO_H */