ixgbe/base: add X550em KR/iXFI internal link mode support

ixgbe/base: add X550em KR/iXFI internal link mode support

This patch adds support for x550em KR/iXFI internal link modes. The initial
x550em-10GBASET and x550em-SFP designs use iXFI internal link mode between
the internal PHY and the external PHY.
However future designs will use a KR internal link. This patch is intended
to future proof the driver by adding the KR internal link support in the
driver now.

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>
Acked-by: Helin Zhang <helin.zhang@intel.com>