sandbox: correct cpu nodes

The cpu nodes in arch/sandbox/dts/test.dts should conform to the devicetree
specification:

* property device_type must be set to "cpu"
* the reg property must be provided
* the cpu nodes must have an address

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>