Include necessary headers explicitly without relying on indirect
header inclusion.  Also, sort them alphabetically.

<linux/delay.h>, <linux/wait.h>, <linux/mutex.h> turned out bogus,
so removed.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>