i2c: Convert i2c-octeon.c to use device tree.

There are three parts to this:

1) Remove the definitions of OCTEON_IRQ_TWSI and OCTEON_IRQ_TWSI2.
   The interrupts are specified by the device tree and these hard
   coded irq numbers block the used of the irq lines by the irq_domain
   code.

2) Remove platform device setup code from octeon-platform.c, it is
   now unused.

3) Convert i2c-octeon.c to use device tree.  Part of this includes
   using the devm_* functions instead of the raw counterparts, thus
   simplifying error handling.  No functionality is changed.

Signed-off-by: David Daney <david.daney@cavium.com>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Cc: linux-mips@linux-mips.org
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/3939/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>