CBMEM: Rename high_tables variables and make them static

Old name was too much x86.
All external references have been removed.

Change-Id: I982b9abfcee57a7ea421c245dadb84342949efae
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/3906
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>

src/include/cbmem.h

@@ -130,7 +130,6 @@ u64 cbmem_entry_size(const struct cbmem_entry *entry);
 #else /* !CONFIG_DYNAMIC_CBMEM */
 
 #ifndef __PRE_RAM__
-extern uint64_t high_tables_base, high_tables_size;
 void set_top_of_ram(uint64_t ramtop);
 void backup_top_of_ram(uint64_t ramtop);
 void cbmem_late_set_table(uint64_t base, uint64_t size);