net/ftgmac100: add a 'aspeed' property

The Aspeed SoCs have a different definition of the end of the ring
buffer bit. Add a property to specify which set of bits should be used
by the NIC.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Jason Wang <jasowang@redhat.com>