powerpc/85xx: Enable I2C_CHARDEV and I2C_MPC options in defconfigs

Enable I2C char dev interface for user space testing of I2C controler.
Enable the I2C driver on 64-bit builds (corenet64_smp_defconfig) as it
was missing.

Signed-off-by: Shaveta Leekha <shaveta@freescale.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>