i2c: Driver to expose PowerNV platform i2c busses

The patch exposes the available i2c busses on the PowerNV platform
to the kernel and implements the bus driver to support i2c and
smbus commands.
The driver uses the platform device infrastructure to probe the busses
on the platform and registers them with the i2c driver framework.

Signed-off-by: Neelesh Gupta <neelegup@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
powerpc/configs: Add I2C_CHARDEV to skiroot_defconfig

Now that we have a i2c bus driver for powernv, enable the chardev.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>