network: set timeline for CONFIG_DM_ETH conversion

The driver model has been supported for network drivers since 2015. It is
time to convert the remaining boards. Set July 2020 as a timeline.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>