clarify PicoDrive's license

- PicoDrive was originally released by fDave with simple
  "free for non-commercial use / For commercial use, separate licencing
  terms must be obtained" license and I kept it in my releases.
- in 2011, fDave re-released his code (same that I used as base
  many years ago) dual licensed with GPLv2 and MAME licenses:
    https://code.google.com/p/cyclone68000/

Based on the above I now proclaim that the whole source code is licensed
under the MAME license as more elaborate form of "for non-commercial use".
If that raises any doubt, I announce that all my modifications (which
is the vast majority of code by now) is licensed under the MAME license,
as it reads in COPYING file in this commit.

This does not affect ym2612.c/sn76496.c that were MAME licensed already
from the beginning.

pico/sound/mix.c

@@ -1,5 +1,10 @@
-// some code for sample mixing
-// (c) Copyright 2006-2007, Grazvydas "notaz" Ignotas
+/*
+ * some code for sample mixing
+ * (C) notaz, 2006,2007
+ *
+ * This work is licensed under the terms of MAME license.
+ * See COPYING file in the top-level directory.
+ */
 
 #define MAXOUT		(+32767)
 #define MINOUT		(-32768)

pico/sound/mix_arm.s

@@ -1,8 +1,10 @@
-@ vim:filetype=armasm
-
-@ Generic routines for mixing audio samples
-@ (c) Copyright 2007, Grazvydas "notaz" Ignotas
-
+/*
+ * Generic routines for mixing audio samples
+ * (C) notaz, 2007,2008
+ *
+ * This work is licensed under the terms of MAME license.
+ * See COPYING file in the top-level directory.
+ */
 
 .text
 .align 4
@@ -364,4 +366,4 @@ m32_16l_st_l_no_unal2:
     ldmfd   sp!, {r4-r9,lr}
     bx      lr
 
-
+@ vim:filetype=armasm

pico/sound/sound.c

@@ -1,11 +1,11 @@
-// This is part of Pico Library
-
-// (c) Copyright 2004 Dave, All rights reserved.
-// (c) Copyright 2006,2007 notaz, All rights reserved.
-// Free for non-commercial use.
-
-// For commercial use, separate licencing terms must be obtained.
-
+/*
+ * PicoDrive
+ * (c) Copyright Dave, 2004
+ * (C) notaz, 2006-2009
+ *
+ * This work is licensed under the terms of MAME license.
+ * See COPYING file in the top-level directory.
+ */
 
 #include <string.h>
 #include "ym2612.h"

pico/sound/ym2612_arm.s

@@ -1,3 +1,11 @@
+/*
+ * PicoDrive
+ * (C) notaz, 2006
+ *
+ * This work is licensed under the terms of MAME license.
+ * See COPYING file in the top-level directory.
+ */
+
 @ this is a rewrite of MAME's ym2612 code, in particular this is only the main sample-generatin loop.
 @ it does not seem to give much performance increase (if any at all), so don't use it if it causes trouble.
 @ - notaz, 2006