doc: frdm_k64f: Document Eth PHY known issue

Working Ethernet, and actually even successfully booting with Ethernet
enabled (and Ethernet is enabled by default on enabling networking)
so far requires a network cable attached.

Change-Id: I9c17e417f867942ddf085db9e8342e945aa07370
Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>