mb/asus/p8h61-m_pro: Relicense gma-mainboard.ads

Commit 65ddbb720b (mb/asus/p8h61-m_pro: Add new mainboard) added this
file, and I authored this commit. Since most gma-mainboard.ads files are
licensed as GPL-2.0-or-later, relicense this one for consistency.

Change-Id: I2d28150f4c97ba600cb46fead7bb29cdc65c5baf
Signed-off-by: Angel Pons <th3fanbus@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/54362
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/mainboard/asus/p8h61-m_pro/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;