board: cogent: include header files in a more natural way

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
diff --git a/board/cogent/serial.c b/board/cogent/serial.c
index 20631d1..f0d6b22 100644
--- a/board/cogent/serial.c
+++ b/board/cogent/serial.c
@@ -4,7 +4,7 @@
  */
 
 #include <common.h>
-#include <board/cogent/serial.h>
+#include "serial.h"
 #include <serial.h>
 #include <linux/compiler.h>