i2c-mpc: Add support for 64bit system

Currently i2c-mpc supports 32bit system only, this modification makes it
supported on both 32-bit and 64-bit systems.  The P5020 is the first
64-bit PPC system with the i2c-mpc controller.

Based in patch from Xulei <B33228@freescale.com>

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>