net: fec: move fec_enet_private to header file

A new file fec_ptp.c will use fec_enet_private to support 1588 PTP
move such structure to common header file fec.h

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