ACPI: remove motherboard driver (redundant with PNP system driver)

The PNP system board driver (drivers/pnp/system.c) contains all the
same functionality, so we don't need the ACPI version.

Previously, a motherboard device would be claimed by *both* the ACPI and
PNP drivers, resulting in stuff like this in /proc/ioports:

    1200-121f : motherboard	<-- from drivers/acpi/motherboard.c
      1200-121f : pnp 00:0d	<-- from drivers/pnp/system.c

Make sure to enable CONFIG_PNP (and CONFIG_PNPACPI) to include the
PNP system board driver.

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