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>