[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>