Do not expose register definitions, structures and helpers via a .h file
because there are used only by at_hdmac.c. Since there are no other users,
remove the ambiguity and move all the .h contents to the .c file.

One may notice some checkpatch warnings and errors with this move. The move
was done "as it was", checkpatch complaints can be fixed in a further
patch.
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20221025090306.297886-1-tudor.ambarus@microchip.com
Link: https://lore.kernel.org/r/20221025090306.297886-28-tudor.ambarus@microchip.comSigned-off-by: Vinod Koul <vkoul@kernel.org>