[SERIAL] Add new port registration/unregistration functions.

[SERIAL] Add new port registration/unregistration functions.

serial8250_register_port()/serial8250_unregister_port() has the
capability of registering ports with their struct device nodes,
which allows sysfs to indicate which tty devices belong to which
hardware devices.

We also add a serial8250 platform device driver in an initial
attempt at PM for ISA ports.  However, I'm leaving out the
platform device for the time being since adding that would cause
potential oops issues.