dm: net: Adjust PHY interface to work with CONFIG_DM_ETH

When driver model is used for Ethernet a few functions are passed a udevice
instead of an eth_device. Also add a function to find a PHY type given its
name. This will be used to decode the device tree node.

Finally, put a phy_interface field in struct eth_pdata since this is an
important part of the platform data for Ethernet.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>