log: Tidy up an ambiguous comment

Add a bit more detail so it is clear that multiple devices are
supported, but only one per driver.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

common/log.c

@@ -437,7 +437,7 @@ int log_init(void)
 	/*
 	 * We cannot add runtime data to the driver since it is likely stored
 	 * in rodata. Instead, set up a 'device' corresponding to each driver.
-	 * We only support having a single device.
+	 * We only support having a single device for each driver.
 	 */
 	INIT_LIST_HEAD((struct list_head *)&gd->log_head);
 	while (drv < end) {