fec: Fix the build as module

Since commit ff43da86 (NET: FEC: dynamtic check DMA desc buff type) the
following build error happens when CONFIG_FEC=m

ERROR: "fec_ptp_init" [drivers/net/ethernet/freescale/fec.ko] undefined!
ERROR: "fec_ptp_ioctl" [drivers/net/ethernet/freescale/fec.ko] undefined!
ERROR: "fec_ptp_start_cyclecounter" [drivers/net/ethernet/freescale/fec.ko] undefined!

Fix it by exporting the required fec_ptp symbols.

Reported-by: Uwe Kleine-Koenig <u.kleine-koenig@pengutronix.de>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>