dmaengine: at_hdmac: trivial: fix comment in header

Not all Atmel SoCs were pointed out in header comment which was bringing
confusion. Remove the truncated list of supported devices, replace by the
only one that is not supported.

Reported-by: Elen Song <elen.song@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>