ethernet: esp32: make phy a phandle of the ethernet device

Change the eth-phy definition so that the phy is pointed by a phandle
rather than a child node, make the phy device a child of mdio. This
makes more sense from a devicetree hirearchy where the phandles have to
be initialized before the device itself, allows keeping the priorities
in check with CHECK_INIT_PRIORITIES.

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