mvebu: Switch to use a module for mv88e6xxx

mvebu: 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,
typically the DSA core is 600 KB so this and some more is saved
for devices with no DSA switch.

When adding the packages I went over both the upstream DTS files
and the OpenWrt-specific DTS files and used grep 'marvell,mv88e6'
to find the devices using these switches.

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