soc/amd/common: Clean up GetHeapBase function

Make GetHeapBase a static function.  Change the type of return value to
a void pointer and remove the unused StdHeader argument.  This should be
innocuous and will allow a subsequent patch to be simpler.

Change-Id: Id4a024d000a514ea9a44f9dfc2caffae9ff01789
Signed-off-by: Marshall Dawson <marshalldawson3rd@gmail.com>
Reviewed-on: https://review.coreboot.org/21593
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/soc/amd/common/BiosCallOuts.h

@@ -35,7 +35,6 @@ typedef struct _BIOS_BUFFER_NODE {
 	UINT32 NextNodeOffset;
 } BIOS_BUFFER_NODE;
 
-UINT32 GetHeapBase(AMD_CONFIG_PARAMS *StdHeader);
 void EmptyHeap(void);
 
 AGESA_STATUS agesa_AllocateBuffer(UINT32 Func, UINTN Data, VOID *ConfigPtr);