Now that the serial driver fully supports discovery via ACPI,
we can get rid of all the legacy junk in asm-ia64/serial.h.
This keeps the serial driver from blindly probing I/O port
space.

This also fixes a long-standing serial device naming issue:
ttyS0-ttyS3 were always reserved for the compiled-in
COM ports, even if they weren't present in the box.
Any additional PCI or ACPI ports appeared starting at
ttyS4 (except for the special case of things described
in the HCDP).   Now we'll just name ttyS devices in the
order they're discovered, without any holes for non-existent
devices.

This is a little bit ugly for serial console because the HCDP
is currently the only way we learn about serial ports before
the actual ACPI discovery.  All HP firmware implements HCDP,
but Intel firmware does not.  So in the absence of an HCDP,
we have to probe for legacy COM ports.  Otherwise, the serial
console wouldn't work until the serial driver discovers the
port via ACPI.