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.ads b/common/broxton/hw-gfx-gma-plls.ads
index d79d378..af957fb 100644
--- a/common/broxton/hw-gfx-gma-plls.ads
+++ b/common/broxton/hw-gfx-gma-plls.ads
@@ -24,7 +24,7 @@
    type T is (Invalid_PLL, DPLL_A, DPLL_B, DPLL_C);
    Invalid : constant T := Invalid_PLL;
 
-   procedure Initialize is null
+   procedure Initialize
    with
       Global => (Output => State);