spl: fix format of function documentation

U-Boot adopted the kernel-doc annotation style.

cc: Michael Trimarchi <michael@amarulasolutions.com>
Signed-off-by: NDario Binacchi <dariobin@libero.it>
Reviewed-by: NSimon Glass <sjg@chromium.org>

include/spl.h

@@ -155,7 +155,7 @@ struct spl_image_info {
 #endif
 };
 
-/*
+/**
  * Information required to load data from a device
  *
  * @dev: Pointer to the device, e.g. struct mmc *