dm: README: recommend u-boot.dtb to try driver-model on sandbox

To enjoy driver-model on sandbox, using device tree is recommended.
While we are here, change sandbox_config to sandbox_defconfig too.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Jagannadha Sutradharudu Teki <jagannadh.teki@gmail.com>