Split serial-isa into its own config option

At present, the core device model code for 8250-like serial ports
(serial.c) and the code for serial ports attached to ISA-style legacy IO
(serial-isa.c) are both controlled by the CONFIG_SERIAL variable.

There are lots and lots of embedded platforms that have 8250-like serial
ports but have never had anything resembling ISA legacy IO.  Therefore,
split serial-isa into its own CONFIG_SERIAL_ISA option so it can be
disabled for platforms where it's not appropriate.

For now, I enabled CONFIG_SERIAL_ISA in every default-config where
CONFIG_SERIAL is enabled, excepting microblaze, or32, and xtensa.  As best
as I can tell, those platforms never used legacy ISA, and also don't
include PCI support (which would allow connection of a PCI->ISA bridge
and/or a southbridge including legacy ISA serial ports).

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>