Improve mainboard.c comments for DBM690T and Pistachio.

Improve mainboard.c comments for DBM690T and Pistachio.
Fix reference to documentation.
Use __FUNCTION__ instead of hardcoding function names in printk
messages.

No functional changes.

I'm slowly getting to the point where adding another RS690 board is
really easy and needs almost no changes to the existing target.

Signed-off-by: Carl-Daniel Hailfinger <c-d.hailfinger.devel.2006@gmx.net>
Acked-by: Myles Watson <mylesgw@gmail.com>
git-svn-id: svn://svn.coreboot.org/coreboot/trunk@3938 2b7e53f0-3cfb-0310-b3e9-8179ed1497e1