cyclic: make clients embed a struct cyclic_info in their own data structure

There are of course not a whole lot of examples in-tree yet, but
before they appear, let's make this API change: Instead of separately
allocating a 'struct cyclic_info', make the users embed such an
instance in their own structure, and make the convention that the
callback simply receives the 'struct cyclic_info *', from which the
clients can get their own data using the container_of() macro.

This has a number of advantages.

First, it means cyclic_register() simply cannot fail, simplifying the
code. The necessary storage will simply be allocated automatically
when the client's own structure is allocated (often via
uclass_priv_auto or similar).

Second, code for which CONFIG_CYCLIC is just an option can more easily
be written without #ifdefs, if we just provide an empty struct
cyclic_info {}. For example, the nested CONFIG_IS_ENABLED()s in
https://lore.kernel.org/u-boot/20240316201416.211480-1-marek.vasut+renesas@mailbox.org/
are mostly due to the existence of the 'struct cyclic_info *' member
being guarded by #ifdef CONFIG_CYCLIC.

And we do probably want to avoid the extra memory overhead of that
member when !CONFIG_CYCLIC. But that is automatic if, instead of a
'struct cyclic_info *', one simply embeds a 'struct cyclic_info',
which will have size 0 when !CONFIG_CYCLIC. Also, the no-op
cyclic_register() function can just unconditionally be called, and the
compiler will see that (1) the callback is referenced, so not emit a
warning for a maybe-unused function and (2) see that it can actually
never be reached, so not emit any code for it.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>

doc/develop/cyclic.rst

@@ -19,20 +19,26 @@ Registering a cyclic function
 
 To register a cyclic function, use something like this::
 
-    static void cyclic_demo(void *ctx)
+    struct donkey {
+        struct cyclic_info cyclic;
+        void (*say)(const char *s);
+    };
+
+    static void cyclic_demo(struct cyclic_info *c)
     {
-        /* Just a small dummy delay here */
-        udelay(10);
+        struct donkey *donkey = container_of(c, struct donkey, cyclic);
+
+        donkey->say("Are we there yet?");
     }
-    
-    int board_init(void)
+
+    int donkey_init(void)
     {
-        struct cyclic_info *cyclic;
-        
+        struct donkey *donkey;
+
+        /* Initialize donkey ... */
+
         /* Register demo cyclic function */
-        cyclic = cyclic_register(cyclic_demo, 10 * 1000, "cyclic_demo", NULL);
-        if (!cyclic)
-        printf("Registering of cyclic_demo failed\n");
+        cyclic_register(&donkey->cyclic, cyclic_demo, 10 * 1000, "cyclic_demo");
         
         return 0;
     }