[vim][ethernet] Get MAC/PHY info through the metadata

Make Board/MAC driver completely independent. Get
MAC & PHY information from the top level board
driver using metadata.

Test: Netboot on vim2

Change-Id: I566e2e418ecb489ec8bce7420c9fff8d38cf3b1c

[vim][ethernet] Add register_callbacks() for PHYs

Adding a register_callbacks() ops. to ZX_PROTOCOL_ETH_MAC.
This allows PHYs to register callbacks for PHY operations lik
ConfigPhy() etc.

Test: Netboot on vim2

Change-Id: I5e5b0f7f89e6940146696d90812ad7418305bac8

[vim][ethernet] Re-write ETH driver using new design

This CL breaks the Ethernet driver into 3 parts
1. ETH_BOARD: This driver has all the board specific
initialization and ops. Currently this driver
loads the DWMAC driver and it's hard coded.
In my next CL I will be getting that information
through the metadata.
2. ETH_MAC: This driver has all the MAC specific init and
ops. Same as above, currently hard coded to load
a particular PHY driver. In my next CLs I will get
that information from the metadata and also be adding
new ops. for PHY to register callbacks.
Currently the PHY config part is part of this driver,
it will be cleaned up in next CL.
Also need to add new ops. such that we support
multiple PHY's.
3. ETH_PHY: This is the PHY implementation. Currently it's not doing
much. In the next CL, we will be registering the callbacks
so MAC driver can call PHY APIs.

Design doc:
https://docs.google.com/document/d/1zoqTEJhWtvrQjUcE6pJ1JoaNpceU9nuCA7L9UuOMHso/edit#heading=h.xgjl2srtytjt

ZX-2686 #done

Test: Netboot on VIM2

Change-Id: I3e7202871d4649fa8139627d6d63b8d1f78f6775