mfd: at91-usart: Include Device Tree header

Will ensure 'of_device_id' is declared.
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/at91-usart.c

@@ -12,6 +12,7 @@
 
 #include <linux/module.h>
 #include <linux/mfd/core.h>
+#include <linux/of.h>
 #include <linux/property.h>
 
 static struct mfd_cell at91_usart_spi_subdev = {