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>

doc/driver-model/README.txt

@@ -36,9 +36,9 @@ How to try it
 
 Build U-Boot sandbox and run it:
 
-   make sandbox_config
+   make sandbox_defconfig
    make
-   ./u-boot
+   ./u-boot -d u-boot.dtb
 
    (type 'reset' to exit U-Boot)