sandbox: eth-raw: Add a SIMPLE_BUS to enumerate host interfaces

Ask the OS for each of its interfaces and for each one, bind a U-Boot
device and then probe it. This will allocate the priv data structure
that is then populated.

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