gianfar: Exclude PPC specific errata handling from ARM builds

This excludes the PPC specific instructions for PPC based SoC
(MPC85xx family) version identification from ARM builds.
The PPC specific macro mfspr() from asm/reg.h is not defined
by the ARM architecture.

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