spi: mediatek: add linux/io.h include file

mediatek spi driver uses readl/writel, so add linux/io.h,
even so it's implicitly imported by spi/spi.h
Signed-off-by: Leilk Liu <leilk.liu@mediatek.com>
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/spi/spi-mt65xx.c

@@ -16,6 +16,7 @@
 #include <linux/device.h>
 #include <linux/err.h>
 #include <linux/interrupt.h>
+#include <linux/io.h>
 #include <linux/ioport.h>
 #include <linux/module.h>
 #include <linux/of.h>