gma broxton: Move `null` implementation of procedure

Newer compilers say the `Global` annotation of the null procedure is
illegal. However, we need that annotation to have a stable interface
for multiple platforms.

Move the "implementation" into the body file, so we are not bothered
by the compiler a little longer.

Change-Id: I4b6f8a5af64097b394fabe4d9e1a735650615fa8
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/libgfxinit/+/55424
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
diff --git a/common/broxton/hw-gfx-gma-plls.adb b/common/broxton/hw-gfx-gma-plls.adb
index 087ff12..6093320 100644
--- a/common/broxton/hw-gfx-gma-plls.adb
+++ b/common/broxton/hw-gfx-gma-plls.adb
@@ -398,6 +398,8 @@
 
    ----------------------------------------------------------------------------
 
+   procedure Initialize is null;
+
    procedure Alloc
      (Port_Cfg : in     Port_Config;
       PLL      :    out T;