ixgbe: add support for interrupts from X550 external PHY

This patch adds support for receiving interrupts from a external copper
PHY for the X550 part.  This includes enabling, detection as well as
re-enablement.

Signed-off-by: Don Skidmore <donald.c.skidmore@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>