net: fec: default select FEC_PTP at mx6 platform

Remove PPS.
Limit FEC_PTP option for i.MX chip only.
FEC_PTP default is on at mx6 platform.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Acked-by: Ben Hutchings <bhutchings@solarflare.com>
Signed-off-by: David S. Miller <davem@davemloft.net>