exynos: Drop old exynos5420-specific board code

Now that exynos5420 boards can use the generic exynos5 code, switch over to
it and remove the old code.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Przemyslaw Marczak <p.marczak@samsung.com>

include/configs/exynos5420-common.h

@@ -13,6 +13,8 @@
 /* A variant of Exynos5420 (Exynos5 Family) */
 #define CONFIG_EXYNOS5800
 
+#define CONFIG_EXYNOS5_DT
+
 #define MACH_TYPE_SMDK5420	8002
 #define CONFIG_MACH_TYPE	MACH_TYPE_SMDK5420