net: fec_mxc: Adjust RX DMA alignment for mx6solox

mx6solox has a requirement for 64 bytes alignment for RX DMA transfer.
Other SoCs work with the standard 32 bytes alignment.

Adjust it accordingly by using 64 bytes aligment in the FEC RX DMA buffers,
which addresses the needs from mx6solox and also works for the other SoCs.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Marek Vasut <marex@denx.de>