edac, highbank: Add Calxeda ECX-2000 support

Implement edac support for Calxeda ECX-2000.

The ECX-2000 memory controller is similar to Highbank but has
different register bases for error and interrupt registers. There is
an own device tree name "calxeda,ecx-2000-ddr-ctrl" for identification
and initialization of the ECX-2000 and its base addresses.

Signed-off-by: Robert Richter <robert.richter@linaro.org>
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Robert Richter <rric@kernel.org>