igb: add IGB_DCA instead of selecting INTEL_IOATDMA

Add a bool IGB_DCA defined to y if IGB and DCA are enabled, but IGB isn't y while DCA=m.  And thus remove the need to select INTEL_IOATDMA when IGB is enabled, so that non-x86 architectures can build the igb driver.
Based on work/patch from Brice Goglin <brice@myri.com>

Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>