[SERIAL] removing legacy UART cruft

[SERIAL] removing legacy UART cruft

I'm trying to remove all the legacy UART cruft from ia64.  In theory
we should be able to discover all UARTs on ia64 via ACPI and PCI
enumeration, so we shouldn't need SERIAL_PORT_DFNS.

But SERIAL_PORT_DFNS currently does two things:
        1) it tells you about legacy devices you can't discover
           via a standard enumeration method, and
        2) it sizes old_serial_port[], which determines UART_NR,
           which is the maximum number of ports the driver will
           support

So here's a proposal to make SERIAL_PORT_DFNS optional and
provide another mechanism for configuring the number of ports
to support.