sandbox: fix building with CONFIG_SPL_TIMER=y

Building sandbox_defconfig with CONFIG_SPL_TIMER=y results in an error

    include/dm/platdata.h:63:33: error: static assertion failed:
    "Cannot use U_BOOT_DRVINFO with of-platdata.
    Please use devicetree instead"

Add a missing condition in the sandbox driver.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>