amd/geode_lx: make done_cache_as_ram_main global

Current code written in C is calling a function implemented
in assembly. However, the symbol's visibility is not set
for such usage. Of course this works because MAINBOARDDIR/romstage.c
is being processed into an assembly file currently.

BUG=chrome-os-partner:44827
BRANCH=None
TEST=Built digitallogic/msm800sev while not changing romstage.c
     into an assembly file.

Change-Id: I84c3af0026f3f98bc64af007aa7cc196429f4e5f
Signed-off-by: Aaron Durbin <adubin@chromium.org>
Reviewed-on: http://review.coreboot.org/11511
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

src/cpu/amd/geode_lx/cache_as_ram.inc

@@ -186,6 +186,7 @@ DCacheSetupGood:
 
 	/* Call romstage.c main function */
 	call	main
+.global done_cache_as_ram_main
 done_cache_as_ram_main:
 
 	/* We now run over the stack-in-cache,