serial: imx: Use <linux/io.h> instead of <asm/io.h>

Silences the following checkpatch warning:
WARNING: Use #include <linux/io.h> instead of <asm/io.h>

Cc: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/imx.c

@@ -48,8 +48,8 @@
 #include <linux/of.h>
 #include <linux/of_device.h>
 #include <linux/pinctrl/consumer.h>
+#include <linux/io.h>
 
-#include <asm/io.h>
 #include <asm/irq.h>
 #include <linux/platform_data/serial-imx.h>