rockchip: evb-rk3339: Enable DHCP

This is the only RK3399 device without DHCP. Enable it so that we
can use a common BOOT_TARGET_DEVICES setting. It is likely useful to be
able to use USB networking, at least. Full networking can be enabled when
a suitable platform needs it.

Signed-off-by: Simon Glass <sjg@chromium.org>