[POWERPC] 4xx: enable built-in networking for Sequoia defconfig

Enable EMAC driver for Sequoia (and while we're in there, disable
Macintosh drivers for Sequoia and Bamboo).

Signed-off-by: Hollis Blanchard <hollisb@us.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>