VIA cpus: apply un-written naming rules

VIA cpus: apply un-written naming rules

Rename files and directories:
  model_c3 -> c3
  model_c7 -> c7

Change-Id: If144fc501e8ae44b347ac44fa90c689c33a8e126
Signed-off-by: Kyösti Mälkki <kyosti.malkki@gmail.com>
Reviewed-on: http://review.coreboot.org/614
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>