of: add const to struct *of_device_id.data

Drivers should never need to modify the data of a device id. So it can
be const which in turn allows more consts in the driver.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

include/linux/mod_devicetable.h

@@ -232,7 +232,7 @@ struct of_device_id
 	char	type[32];
 	char	compatible[128];
 #ifdef __KERNEL__
-	void	*data;
+	const void *data;
 #else
 	kernel_ulong_t data;
 #endif