net: fec: change FEC alignment according to i.mx6 sx requirement

i.MX6 SX change FEC alignment requirement.
i.MX6 SX change internal bus from AHB to AXI.
It require RX buffer must be 64 bytes alignment.
And remove TX buffer alignment requirement.

Signed-off-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Frank Li <Frank.Li@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>