log: Add a way to enable/disable a log device

At present all log devices are enabled by default. Add a function to allow
devices to be disabled or enabled at runtime.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/log.h b/include/log.h
index d28bc1e..4acc087 100644
--- a/include/log.h
+++ b/include/log.h
@@ -388,6 +388,10 @@
 #define LOG_DRIVER(_name) \
 	ll_entry_declare(struct log_driver, _name, log_driver)
 
+/* Get a pointer to a given driver */
+#define LOG_GET_DRIVER(__name)						\
+	ll_entry_get(struct log_driver, __name, log_driver)
+
 /**
  * log_get_cat_name() - Get the name of a category
  *
@@ -465,6 +469,19 @@
  */
 int log_remove_filter(const char *drv_name, int filter_num);
 
+/**
+ * log_device_set_enable() - Enable or disable a log device
+ *
+ * Devices are referenced by their driver, so use LOG_GET_DRIVER(name) to pass
+ * the driver to this function. For example if the driver is declared with
+ * LOG_DRIVER(wibble) then pass LOG_GET_DRIVER(wibble) here.
+ *
+ * @drv: Driver of device to enable
+ * @enable: true to enable, false to disable
+ * @return 0 if OK, -ENOENT if the driver was not found
+ */
+int log_device_set_enable(struct log_driver *drv, bool enable);
+
 #if CONFIG_IS_ENABLED(LOG)
 /**
  * log_init() - Set up the log system ready for use