dm: exynos: Tidy up GPIO headers

The wrong header is being included, thus requiring the code to re-declare
the generic GPIO interface in each GPIO header.

Fix this.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/board/samsung/smdk5420/smdk5420.c b/board/samsung/smdk5420/smdk5420.c
index 270ee83..47998ab 100644
--- a/board/samsung/smdk5420/smdk5420.c
+++ b/board/samsung/smdk5420/smdk5420.c
@@ -11,9 +11,9 @@
 #include <lcd.h>
 #include <spi.h>
 #include <errno.h>
+#include <asm/gpio.h>
 #include <asm/arch/board.h>
 #include <asm/arch/cpu.h>
-#include <asm/arch/gpio.h>
 #include <asm/arch/pinmux.h>
 #include <asm/arch/system.h>
 #include <asm/arch/dp_info.h>