cpu: sandbox: Add "u-boot, dm-pre-reloc" for all cpu nodes

To support CONFIG_DISPLAY_CPUINFO, add "u-boot,dm-pre-reloc" for
all cpu nodes in Sandbox test.dts.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>