mb/lenovo/t420/gma-mainboard.ads: Replace with GPLv2+ equivalent

Replace it with t420s/gma-mainboard.ads which is licensed under more
flexible terms (GPL-2.0-only vs. GPL-2.0-or-later). Apart from licensing
terms these files are identical.

This makes diff between boards smaller.

Change-Id: I5393a603b5a4cd353149c1fa9e3e29020946b962
Signed-off-by: Peter Lemenkov <lemenkov@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41498
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/mainboard/lenovo/t420/gma-mainboard.ads

@@ -1,4 +1,4 @@
--- SPDX-License-Identifier: GPL-2.0-only
+-- SPDX-License-Identifier: GPL-2.0-or-later
 
 with HW.GFX.GMA;
 with HW.GFX.GMA.Display_Probing;