net: fec: use CONFIG_ARM instead of CONFIG_ARCH_MXC/SOC_IMX28

As Arnd Bergmann points out, using CONFIG_ARCH_MXC and/or SOC_IMX28
is wrong if some other ARM platform uses this device - the operation
of the driver would depend on an unrelated ARM platform that might
or might not be set for multi-platform kernels.

Prior to my previous patch, any other platforms using it would have
been broken already due to having the cbd_datlen/cbd_sc fields in
the wrong order, but byte ordering correctly, so no such platforms
can exist and work today.

In any case, it seems likely that only Freescale SoCs use this part,
and those are little-endian on ARM, so CONFIG_ARM is safe for them.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>