drivers: nxp_enet: Correct PTP clock dependencies

The dependencies should be in a 'depends on' clause.
Also, 'depends on PTP_CLOCK' is redundant because this is
within 'if PTP_CLOCK' already.

Additionally, conditionally include the PTP header in the mac driver.

Signed-off-by: Declan Snyder <declan.snyder@nxp.com>