i2c-ocores: Adapt for device tree

This patch adapts the i2c-ocores driver for being defined and configured via
a device tree description.

The device tree bits need to be protected by CONFIG_OF guards as this is
still an optional feature.

Signed-off-by: Jonas Bonn <jonas@southpole.se>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>