pinctrl: aspeed: Add missing header(s)

Do not imply that some of the generic headers may be always included.
Instead, include explicitly what we are direct user of.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

drivers/pinctrl/aspeed/pinctrl-aspeed.c

@@ -5,6 +5,7 @@
 
 #include <linux/mfd/syscon.h>
 #include <linux/platform_device.h>
+#include <linux/seq_file.h>
 #include <linux/slab.h>
 #include <linux/string.h>
 #include "../core.h"