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>