[POWERPC] Fix Kconfig undefined symbol 'IBM_NEW_EMAC_ZMII'

An allmodconfig on the current powerpc tree yields:
arch/powerpc/platforms/44x/Kconfig:41:warning: 'select' used by
config symbol '440GP' refers to undefined symbol 'IBM_NEW_EMAC_ZMII'

Hide the select until the driver exists.

Signed-off-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/platforms/44x/Kconfig

@@ -38,7 +38,8 @@ config 440EP
 
 config 440GP
 	bool
-	select IBM_NEW_EMAC_ZMII
+# Disabled until the new EMAC Driver is merged.
+#	select IBM_NEW_EMAC_ZMII
 
 config 440GX
 	bool