mfd: syscon: atmel-smc: Include string.h

The string.h header file is needed for the memset() definition. The RT
build fails because it is not pulled in via other header files.
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/atmel-smc.c

@@ -12,6 +12,7 @@
  */
 
 #include <linux/mfd/syscon/atmel-smc.h>
+#include <linux/string.h>
 
 /**
  * atmel_smc_cs_conf_init - initialize a SMC CS conf