i386: turn on CONFIG_PNP in defconfig

I'm trying to remove drivers/acpi/motherboard.c, which is mostly
redundant with drivers/pnp/system.c.  So make sure that we include the
PNP driver in the default config.  Most distros enable this already.

Turning on CONFIG_PNP also causes the following options to be enabled:

    CONFIG_PNPACPI
    CONFIG_SERIAL_8250_PNP

CONFIG_SERIAL_8250_PNP causes legacy serial ports to be discovered
twice, which is ugly but harmless:

    serial8250: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A
    00:07: ttyS0 at I/O 0x3f8 (irq = 4) is a 16550A

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