drivers: nxp_enet: Add NET DEVICE PM function

Add functionality for when NET_POWER_MANAGEMENT (ie PM_DEVICE)
is enabled. This function code originally comes from the
old eth_mcux driver which was only tested on kinetis family SOCs,
so that family kconfig is a dependency for this feature for now.

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