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>
diff --git a/drivers/timer/sandbox_timer.c b/drivers/timer/sandbox_timer.c
index c846bfb..1da7e0c 100644
--- a/drivers/timer/sandbox_timer.c
+++ b/drivers/timer/sandbox_timer.c
@@ -66,6 +66,8 @@
 };
 
 /* This is here in case we don't have a device tree */
+#if !CONFIG_IS_ENABLED(OF_PLATDATA)
 U_BOOT_DRVINFO(sandbox_timer_non_fdt) = {
 	.name = "sandbox_timer",
 };
+#endif