ibm_newemac: Allow the "no flow control" EMAC feature to work

Some PowerPC 40x chips have errata that force us not to use the integrated
flow control.  We have the feature defined, but it currently can't be used
because it is never added to EMAC_FTRS_POSSIBLE.

This adds a Kconfig option for affected platforms to select and puts the
feature in the EMAC_FTRS_POSSIBLE list.  This is set for PowerPC 405EZ
platforms as well.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Jeff Garzik <jeff@garzik.org>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>