net: Move driver-model code into its own file

Every other uclass is in its own file. Create a new eth-uclass.c file and
move the driver-model code into it, so that networking is consistent.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>