i2c-ibm_iic: Support building as an of_platform driver

This patch allows the i2c-ibm_iic driver to be built either as an ocp 
driver or an of_platform driver. This allows it to run under the powerpc 
arch but maintains backward compatibility with the ppc arch.

Signed-off-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>