drivers: core: nullify gd->dm_root after dm_uninit()

To reset the DM after a new dtb is loaded, we need to call dm_uninit()
and then dm_init(). This fails however because gd->dm_root is not nullified
by dm_uninit().
Fixing it by setting gd->dm_root in dm_uninit().

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heiko Schocher <hs@denx.de>

drivers/core/root.c

@@ -187,6 +187,7 @@ int dm_uninit(void)
 {
 	device_remove(dm_root(), DM_REMOVE_NORMAL);
 	device_unbind(dm_root());
+	gd->dm_root = NULL;
 
 	return 0;
 }