annotate that [fp, #-4] is the saved lr

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>

arch/arm/kernel/entry-common.S

@@ -136,7 +136,7 @@ ENTRY(mcount)
 	ldmia sp!, {r0-r3, pc}
 
 trace:
-	ldr r1, [fp, #-4]
+	ldr r1, [fp, #-4]			@ lr of instrumented routine
 	mov r0, lr
 	sub r0, r0, #MCOUNT_INSN_SIZE
 	mov lr, pc