log: Add an implementation of logging

Add the logging header file and implementation with some configuration
options to control it.

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

include/asm-generic/global_data.h

@@ -114,6 +114,11 @@ typedef struct global_data {
 	struct bootstage_data *bootstage;	/* Bootstage information */
 	struct bootstage_data *new_bootstage;	/* Relocated bootstage info */
 #endif
+#ifdef CONFIG_LOG
+	int log_drop_count;		/* Number of dropped log messages */
+	int default_log_level;		/* For devices with no filters */
+	struct list_head log_head;	/* List of struct log_device */
+#endif
 } gd_t;
 #endif