SERIAL: Enable port-mapped access

The x86 architecture exclusively uses Port-Mapped I/O (inb/outb) to access
the 16550 UARTs. This patch mimics how Linux selects between Memory-Mapped
and Port-Mapped I/O. This allows x86 boards to use CONFIG_SERIAL_MUTLI and
drop the custom serial port driver

Signed-off-by: Graeme Russ <graeme.russ@gmail.com>