ixgbe: Add optional DCA infrastructure

82598 cards and up support DCA, which enables the chipset to warm
up the caches for upcoming payload data. This code makes the
driver plug into the CONFIG_DCA infrastructure that was merged
earlier.

Signed-off-by: Jeb Cramer <cramerj@intel.com>
Signed-off-by: Auke Kok <auke-jan.h.kok@intel.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>