dm: rename device struct to udevice

using UBI and DM together leads in compiler error, as
both define a "struct device", so rename "struct device"
in include/dm/device.h to "struct udevice", as we use
linux code (MTD/UBI/UBIFS some USB code,...) and cannot
change the linux "struct device"

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Marek Vasut <marex@denx.de>

drivers/core/root.c

@@ -24,7 +24,7 @@ static const struct driver_info root_info = {
 	.name		= "root_driver",
 };
 
-struct device *dm_root(void)
+struct udevice *dm_root(void)
 {
 	if (!gd->dm_root) {
 		dm_warn("Virtual root driver does not exist!\n");