dmaengine: Kconfig: fix Atmel at_hdmac entry

Remove SoC dependency and make it generic for every Atmel ARM AT91. That will
allow to select this driver for newer chips. Keep dependency on AT91 because of
the use of an header file located in include/mach directory.
Modify the comment to reflect this.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Vinod Koul <vinod.koul@linux.intel.com>