mx25: Define cpu_eth_init() only if needed

The FEC is the only SoC Ethernet support available on i.MX25, so define
cpu_eth_init() only for it instead of returning a misleading success code.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>
Cc: Stefano Babic <sbabic@denx.de>