configs: at91-sama5_common: fix for CONFIG_AT91_GPIO

Add #ifndef CONFIG_DM_GPIO for CONFIG_AT91_GPIO define to avoid
the redefine compilation error.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas@biessmann.org>
diff --git a/include/configs/at91-sama5_common.h b/include/configs/at91-sama5_common.h
index 3e97948..dbd4d84 100644
--- a/include/configs/at91-sama5_common.h
+++ b/include/configs/at91-sama5_common.h
@@ -27,7 +27,9 @@
 #define CONFIG_ENV_VARS_UBOOT_CONFIG
 
 /* general purpose I/O */
+#ifndef CONFIG_DM_GPIO
 #define CONFIG_AT91_GPIO
+#endif
 
 
 /*