sconfig: add cpu device type

In order to enumerate CPU devices that are non-x86 (read: no lapic)
provide a generic 'cpu' device.

Change-Id: Ifeafdad8076935c3448784e6958117002509acbf
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/6824
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>