eth/mcux: Add temporary workaround to unbreak IPv6 ND features.

This is a workaround for lack of driver API support for multicast
management. So, instead we want to receive all multicast frames
"by default", or otherwise basic IPv6 features, like address
resolution, don't work. On Kinetis Ethernet controller, that
translates to enabling promiscuous mode. The real fix depends
on ZEP-1673.

Change-Id: I98a27584be65bdc405de005383eb30bad2a7fcfc
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>