x86/PCI: make Broadcom CNB20LE driver EMBEDDED and EXPERIMENTAL

This functionality is known to be incomplete, so discourage its use in
general-purpose kernels.

The only reason to use this driver is to support PCI hotplug on CNB20LE-
based machines that don't have ACPI, and there are very few such
systems.

Reference: https://bugzilla.redhat.com/show_bug.cgi?id=665109
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>