lib: Rename devtree_update to mb_devtree_update

Rename the devtree_update() bootstate hook added in commit f8494fbeae
("lib: Add devtree_update bootstate hook") to mb_devtree_update()
for clarity, since it is a mainboard-provided hook.

Update all declarations, definitions, and call sites accordingly.

TEST=build Starlabs Starfighter MTL

Change-Id: Id7fd9811433a668905d8439b90a8ee34a472d117
Signed-off-by: Matt DeVillier <matt.devillier@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/91570
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/include/devtree_update.h

@@ -9,6 +9,6 @@
  *
  * This function is called early in ramstage at BS_PRE_DEVICE.
  */
-void devtree_update(void);
+void mb_devtree_update(void);
 
 #endif /* __DEVTREE_UPDATE_H__ */