ARM: dts: calxeda: move memory-controller node out of ecx-common.dtsi

The DDR controller is slightly different in ECX-2000 and ECX-1000, so we
need to have different nodes for each platform.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
[Device Tree documentation updated.]
Signed-off-by: Robert Richter <rric@kernel.org>