ibm_newemac: Cleanup/fix support for STACR register variants

There are a few variants of the STACR register that affect more than
just the "AXON" version of EMAC. Replace the current test of various
chip models with tests for generic properties in the device-tree.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Stefan Roese <sr@denx.de>
Signed-off-by: Jeff Garzik <jeff@garzik.org>