VIA southbridge K8T890: Apply un-written naming rules

VIA southbridge K8T890: Apply un-written naming rules

Use separate Kconfig option to select a driver directory for
build and the specific type of southbridge to support.

Change-Id: I9482d4ea0f0234b9b7ff38144e45022ab95cf3f3
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/685
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>