Various 440BX and Tyan S1846 related minor changes and fixes (trivial):

Various 440BX and Tyan S1846 related minor changes and fixes (trivial):

 - Only check the RAM from 0 - 640 KB and 768 KB - 1 MB now. That's
   available on all boards, regardless of what DIMMs you use.
   Tested on the Tyan S1846, works fine.

 - Properly set the PAM registers to allow the region from 768 KB - 1 MB
   to be used as normal RAM (required for the above).

 - Document all of this properly. Add/improve other documentation, too.

 - Simplify and document code in northbridge.c.

 - Cosmetics and coding style.

Signed-off-by: Uwe Hermann <uwe@hermann-uwe.de>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@2701 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1