powerpc: fix i2c on PPC linkstation / kurobox machines

The i2c bus defn is broken on linkstation / kurobox machines since at
least 2.6.27. Fix it. Also remove CONFIG_SERIAL_OF_PLATFORM, which, if
enabled, breaks the serial console after the
"console handover: boot [udbg0] -> real [ttyS1]" message.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>