net: dpaa2: improve PTP Kconfig option

Converted to use "imply" instead of "select" for PTP_1588_CLOCK
driver selecting. This could break the hard dependency between
the PTP clock subsystem and ethernet drivers.
This patch also set "default y" for dpaa2 ptp driver building to
provide user an available ptp clock in default.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>