PNPACPI: remove EXPERIMENTAL dependency

PNPACPI is pretty widely used and seems fairly stable, so remove the
dependency on EXPERIMENTAL.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Cc: Adam Belay <ambx1@neo.rr.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>