net: fec: forbid FEC_PTP on SoCs that do not support

Beside imx6q, the kernel built from imx_v6_v7_defconfig is also
supposed to be running on other IMX SoCs that do not have the PTP
block.  Before fec driver gets fixed to run-time detect target hardware
rather than conditional compiling with #ifdef CONFIG_FEC_PTP, let's
give it a quick fix in Kconfig to forbid FEC_PTP on those IMX SoCs that
do not support PTP.

Reported-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>