net: mvpp2: MVPP2 now needs MVMDIO

Changes to mvpp2.c require the MVMDIO module which in turn uses
DM_MDIO.

Signed-off-by: Nevo Hed <nhed+github@starry.com>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>