AGESA f14: Add "const" modifiers

Apply commit 283ba78415 to f14 (literally, plus one adaptation).

Change-Id: Ieea47470e5852ec8a46596ce23a2d18444618624
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/7361
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>