Add missing include of model_6bx for slot_1.

Add missing include of model_6bx for slot_1.

I could no longer boot my P3B-F with my Tualeron and r5938. Dies with
"unknown CPU". I believe it will happen with any Slot 1 440BX boards
that supports model_6bx CPUs.

I need to make the change below to make it work. abuild tested. Boot
tested on P2B-LS and P3B-F.

Signed-off-by: Keith Hui <buurin@gmail.com>
Acked-by: Uwe Hermann <uwe@hermann-uwe.de>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@5945 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1