dm: core: Don't allow uclass use before ready

At present it is possible to call uclass_get() before driver model is
inited. In fact this happens on x86 boards which use Intel FSPv1, since
mrccache_get_region() tries to get the SPI flash device very early
during init.

This has always been undefined behaviour. Previously it generally worked,
i.e. returned an error code without crashing, because gd->uclass_root_s
is zeroed and the uclass can be added despite driver model not being
ready, due to the way lists are implemented. With the change to use a
gd->uclass_root pointer, this no-longer works. For example, it causes a
hang on minnowmax.

Fix this by adding a check that driver model is ready when uclass_get() is
called. This function is called in the process of locating any device, so
it is a good place to add the check.

This fixes booting on minnowmax.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: 8a715530bb ("dm: core: Allow the uclass list to move")

include/dm/uclass.h

@@ -159,7 +159,8 @@ void *uclass_get_priv(const struct uclass *uc);
  *
  * @key: ID to look up
  * @ucp: Returns pointer to uclass (there is only one per ID)
- * @return 0 if OK, -ve on error
+ * @return 0 if OK, -EDEADLK if driver model is not yet inited, other -ve on
+ *	other error
  */
 int uclass_get(enum uclass_id key, struct uclass **ucp);