modules: Add kernel module for MV88E6xxx DSA switch

This adds a kernel module package for the Marvell
MV88E6XXX DSA switch and a separate module package for
the DSA tagger since it can in theory be used by multiple
DSA switches. Enable both DSA and EDSA tags in the
tagger.

We can't just compile this in because just a few devices
has this DSA, and it depends on e.g. the I2C and SFP
to be loaded as modules first.

We have no examples of DSA switches being packaged as
modules before, all seem to be compiled in, but it
actually works just fine to do this.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>