powerpc/i2c: Convert i2c-mpc into an of_platform driver

Convert i2c-mpc to an of_platform driver. Utilize the code in
drivers/of-i2c.c to make i2c modules dynamically loadable by the
device tree.

Signed-off-by: Jon Smirl <jonsmirl@gmail.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>