dmaengine: at_hdmac: use subsys_initcall instead of module_init

Use subsys_initcall instead of module_init in order to keep DMA engine rolling
before other peripheral drivers.

Signed-off-by: Eric Xu <hong.xu@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>