net: dsa: loop: Add module soft dependency

There is a soft dependency against dsa_loop_bdinfo.ko which sets up the
MDIO device registration, since there are no symbols referenced by
dsa_loop.ko, there is no automatic loading of dsa_loop_bdinfo.ko which
is needed.

Fixes: 98cd1552 ("net: dsa: Mock-up driver")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>