sandbox: Correct dtc warning in /chosen node

sandbox: Correct dtc warning in /chosen node

At present dtc produces these warnings when compiling sandbox:

arch/sandbox/dts/test.dtb: Warning (reg_format): "reg" property in /chosen/chosen-test has invalid length (8 bytes) (#address-cells == 2, #size-cells == 1)
arch/sandbox/dts/test.dtb: Warning (avoid_default_addr_size): Relying on default #address-cells value for /chosen/chosen-test
arch/sandbox/dts/test.dtb: Warning (avoid_default_addr_size): Relying on default #size-cells value for /chosen/chosen-test

Add the missing properties to avoid this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: f200680 (dm: core: parse chosen node)