ixgbe/base: add register definition for SGMII busy

ixgbe/base: add register definition for SGMII busy

The X550EM_a device provides the MAC_SGMII_BUSY register to
indicate when slow SGMII register writes complete. Add
definitions for the register. No definitions are provided for
the individual bits under the theory that it is better to wait
for everything to complete when needed rather than try to map
out which reads need to wait for which writes. So we should wait
when anything is marked as "busy".

Signed-off-by: Wenzhuo Lu <wenzhuo.lu@intel.com>