dmaengine: at_hdmac: remove platform data header

linux/platform_data/dma-atmel.h is only used by the at_hdmac driver. Move
the CFG bits definitions back in at_hdmac_regs.h and the remaining
definitions in the driver.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20201228203022.2674133-1-alexandre.belloni@bootlin.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>