CBMEM AMD: Remove references to global high_tables_base

Prepare for removal of globals high_tables_base and _size
by replacing the references with a helper function.

Added set_top_of_ram_once() may be called several times,
but only the first call (with non-zero argument) takes effect.

Change-Id: I5b5f71630f03b6a01e9c8ff96cb78e9da03e5cc3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3894
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/include/cbmem.h

@@ -136,6 +136,7 @@ void cbmem_add_lb_mem(struct lb_memory *mem);
 
 #ifndef __PRE_RAM__
 extern uint64_t high_tables_base, high_tables_size;
+void set_top_of_ram_once(uint64_t ramtop);
 void set_cbmem_toc(struct cbmem_entry *);
 #endif