gianfar: Fix stashing support

Stashing is only supported on the 85xx (e500-based) SoCs.  The 83xx and 86xx
chips don't have a proper cache for this.  U-Boot has been updated to add
stashing properties to the device tree nodes of gianfar devices on 85xx.  So
now we modify Linux to keep stashing off unless those properties are there.

Signed-off-by: Andy Fleming <afleming@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>