dmaengine: at_hdmac: add a few const qualifiers

dmaengine: at_hdmac: add a few const qualifiers

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

	drivers/dma/at_hdmac.c: In function 'at_dma_get_driver_data':
	drivers/dma/at_hdmac.c:1228: warning: return discards qualifiers from pointer target type

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>