event: Add basic support for events

Add a way to create and dispatch events without needing to allocate
memory. Also add a way to 'spy' on events, thus allowing 'hooks' to be
created.

Use a linker list for static events, which we can use to replace functions
like arch_cpu_init_f(). Allow an EVENT_DEBUG option which makes it
easier to see what is going on at runtime, but uses more code space.

Dynamic events allow the creation of a spy at runtime. This is not always
necessary, but can be enabled with EVENT_DYNAMIC if needed.

A 'test' event is the only option for now.

Signed-off-by: Simon Glass <sjg@chromium.org>

include/asm-generic/global_data.h

@@ -20,6 +20,7 @@
  */
 
 #ifndef __ASSEMBLY__
+#include <event_internal.h>
 #include <fdtdec.h>
 #include <membuff.h>
 #include <linux/list.h>
@@ -467,6 +468,12 @@ struct global_data {
 	 */
 	char *smbios_version;
 #endif
+#if CONFIG_IS_ENABLED(EVENT)
+	/**
+	 * @event_state: Points to the current state of events
+	 */
+	struct event_state event_state;
+#endif
 };
 #ifndef DO_DEPS_ONLY
 static_assert(sizeof(struct global_data) == GD_SIZE);
@@ -532,6 +539,12 @@ static_assert(sizeof(struct global_data) == GD_SIZE);
 #define gd_set_multi_dtb_fit(_dtb)
 #endif
 
+#if CONFIG_IS_ENABLED(EVENT_DYNAMIC)
+#define gd_event_state()	((struct event_state *)&gd->event_state)
+#else
+#define gd_event_state()	NULL
+#endif
+
 /**
  * enum gd_flags - global data flags
  *