net/ibm/emac: add 8192 rx/tx fifo size

emac4syn chips has availability to use 8192 rx/tx fifo buffer sizes,
in current state if we set it up in dts 8192 as example, we will get
only 2048 which may impact on network speed.

Signed-off-by: Ivan Mikhaylov <ivan@de.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>