Make PCI CONF2 support a compile time option

Make PCI CONF2 support a compile time option

It's not used on any board supported by coreboot but has been
detected at run time since ages. No new boards (since 2000?)
are using the CONF2 method, so it is unlikely we ever have to
turn this on for a board.

Change-Id: I17df94a8a77b9338fde10a6b114b44d393776e66
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/758
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>