qoriq: Switch to use a module for mv88e6xxx

qoriq: Switch to use a module for mv88e6xxx

It's possible to add the driver for the Marvell MV88E6xxx DSA
switches using a module package rather than to compile it into
the kernel. For affected devices this saves a bit of space,
the DSA core alone is around 600 KB on ARM.

I could only find one device actually using this switch (I also
checked upstream DTS files) so I have added the package to that
one device.

In the config CONFIG_NET_DSA_TAG_TRAILER and CONFIG_NET_DSA_OCELOT
were also selected, which seems like mistakes. These taggers are
only used by the MV88E6060 and driver which is a separate switch
from MV88E6xxx and the Ocelot drivers such as
CONFIG_NET_DSA_MSCC_FELIX or CONFIG_NET_DSA_MSCC_SEVILLE
and no qoric platform seems to be using them, nor are they
selected in the config.

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