[POWERPC] Disable old EMAC driver in arch/powerpc

The EMAC driver, in drivers/net/ibm_emac, for the embedded Ethernet
MAC found in PowerPC 4xx embedded chips is not suitable for
arch/powerpc.  It will not build because it relies on the old arch/ppc
OCP mechanism.  BenH has a new, device-tree aware version of the
driver which will work in arch/powerpc, but until it's merged, this
patch will disable the old, non-building version.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>