Add support for RDC R8610 Southbridge

So far it just setups things right for Bifferboard. We may change it
in the future to fit other hardware.

Change-Id: I1c4ccff4e47b9cb9e31a738f038fc4f4ebe59087
Signed-off-by: Rudolf Marek <r.marek@assembler.cz>
Reviewed-on: http://review.coreboot.org/808
Tested-by: build bot (Jenkins)
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>