rockchip: config: Enable the 'gpio' command

Now that we have a pretty good GPIO driver, enable the 'gpio' command on all
rockchip boards.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/configs/rk3288_common.h

@@ -79,6 +79,7 @@
 
 #define CONFIG_CMD_CACHE
 #define CONFIG_CMD_TIME
+#define CONFIG_CMD_GPIO
 
 #define CONFIG_SYS_SDRAM_BASE		0
 #define CONFIG_NR_DRAM_BANKS		1