NET: FEC: dynamtic check DMA desc buff type

MX6 and mx28 support enhanced DMA descriptor buff to support 1588
ptp. But MX25, MX3x, MX5x can't support enhanced DMA descriptor buff.
Check fec type and choose correct DMA descriptor buff type.

Remove static config CONFIG_FEC_PTP.
ptp function will be auto detected.

Signed-off-by: Frank Li <Frank.Li@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>