gianfar: Use mpc85xx support for errata detection

Use the macros and defines from mpc85xx.h to simplify
and prevent errors in identifying a mpc85xx based SoC
for errata detection.
This should help enabling (and identifying) workarounds
for various mpc85xx based chips and revisions.
For instance, express MPC8548 Rev.2 as:
(SVR_SOC_VER(svr) == SVR_8548) && (SVR_REV(svr) == 0x20)
instead of:
(pvr == 0x80210020 && mod == 0x8030 && rev == 0x0020)

Signed-off-by: Claudiu Manoil <claudiu.manoil@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>