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>
11 files changed