mainboard.c: Fix typo in appro*p*riate in comment

Use the following command to fix all occurences.

	$ git grep -l approriate | xargs sed -i 's/approriate/appropriate/g'

Change-Id: I4cbba972bb445c2407ef2e63ffb3068fc948f1c6
Signed-off-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-on: http://review.coreboot.org/5987
Tested-by: build bot (Jenkins)
Reviewed-by: Kyösti Mälkki <kyosti.malkki@gmail.com>