ACPI: always enable CONFIG_PNPACPI on CONFIG_ACPI kernels

We removed the ACPI motherboard driver which handled
the ACPI=y, PNP=n case, so now we need to enforce that
PNP & PNPACPI are always enabled for ACPI kernels.

Most major distros ship this way this already.

Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>