gianfar: add support for SGMII

Add code for initialising and configuring TBI interface and
programming it for connecting to on-chip SERDES (Lynx PHY)
in case of SGMII mode selected through HRCW at reset.

also add defines for TBI register configuration. TBI
interface is programmed towards the SERDES.

refactored mdio read/write functions to differentiate
programming local interface MII regs (e.g., for TBI) from
always programming the mdio master (TSEC1, for programming
the PHYs).

Signed-off-by: Kapil Juneja <Kapil.Juneja@freescale.com>
Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>