parisc: Fix comment on Elf64 function descriptor

The format of the Elf64 function descriptor is defined by the ABI.
Mention the various use cases in the comment.
Signed-off-by: Helge Deller <deller@gmx.de>

arch/parisc/include/asm/elf.h

@@ -163,8 +163,7 @@ typedef struct elf32_fdesc {
 
 /* Format for the Elf64 Function descriptor */
 typedef struct elf64_fdesc {
-	__u64	dummy[2]; /* FIXME: nothing uses these, why waste
-			   * the space */
+	__u64	dummy[2]; /* used by 64-bit eBPF and tracing functions */
 	__u64	addr;
 	__u64	gp;
 } Elf64_Fdesc;