eth/eth_mcux: Make promiscous mode configurable.

Provide a configuration option for promiscuous mode.

Promiscuous mode provides a convenient workaround for ZEP-1673 however
it generates significant demand for RX buffers on a loaded network.
Add a configuration option to conveniently enable / disable.

The Kconfig defaults promiscuous on to workaroudn 1673, once that
issue is resolved the default logic on NET_IPV6 will be dropped.

Change-Id: I6929aca70d7bd88ce88c65d6654d664ea6653b66
Signed-off-by: Marcus Shawcroft <marcus.shawcroft@arm.com>