ethernet: sam-gmac: make phy a phandle of the ethernet device

Make ethernet phys childs of the mdio device and move the mdio device up
a level on the tree. That makes the device hierarchy coherent with the
required initialization priority and allows keeping the sequence in
check with CHECK_INIT_PRIORITIES.

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>