net: fec: use a more proper compatible string for MVF type device

MVF is a family while MVF600 is a particular SoC in the family.  We
generally prefer to use SoC rather than family name in compatible string
to define a particular type of fec device.  And this is how fec_dt_ids
works for all those IMX fec variants.  Let's change mvf to mvf600 to
have it work in the same way.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>