Switch to using the device tree to register serial ports.

Add all the ports with compatible = "cavium,octeon-3860-uart".  Octeon serial
ports have their own device type, required port flags, and I/O
functions, so using of_serial.c is not indicated.

We need to do this as late_initcall, as the 8250 driver must be
initialized before we add any ports.  8250 initialization is done at
device_initcall time.

The OCTEON_IRQ_UART{0,1,2} symbols are removed as they are now unused
and interfere with irq_domain used by the device tree code.
Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Cc: linux-mips@linux-mips.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Rob Herring <rob.herring@calxeda.com>
Cc: linux-kernel@vger.kernel.org
Cc: David Daney <david.daney@cavium.com>
Patchwork: https://patchwork.linux-mips.org/patch/3942/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>