misc/atmel_tc: make atmel_tc.tcb_config member point to const data

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/misc/atmel_tclib.c: In function 'tc_probe':
	drivers/misc/atmel_tclib.c:170: warning: assignment discards qualifiers from pointer target type
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>

include/linux/atmel_tc.h

@@ -63,7 +63,7 @@ struct atmel_tc {
 	struct platform_device	*pdev;
 	struct resource		*iomem;
 	void __iomem		*regs;
-	struct atmel_tcb_config	*tcb_config;
+	const struct atmel_tcb_config *tcb_config;
 	int			irq[3];
 	struct clk		*clk[3];
 	struct list_head	node;