Refine physical address mapping of flash chips.

 - Create distinct functions for mapping and unmapping for flash chips.
 - Map only when needed: map before probing and unmap immediately
   after it. Map again when a single chip was probed successfully before
   taking any actual actions and clean up afterwards.
 - Map special function chip registers centrally together with flash space
   instead of within (some) probing methods after successful probes.
 - Save the used base addresses of the mappings in struct flashctx as well.
 - Do not try to (un)map the zero-sized chip definitions that are merely hacks.
   This also fixes the printing of wrong warnings for these chip definitions
   introduced in r1765.

Signed-off-by: Stefan Tauner <stefan.tauner@alumni.tuwien.ac.at>
Acked-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>

git-svn-id: svn://flashrom.org/flashrom/trunk@1847 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1