of: Document long-ram-code property in nvidia,tegra20-apbmisc

Needed to properly decode the RAM code register.
Signed-off-by: NTomeu Vizoso <tomeu.vizoso@collabora.com>
Signed-off-by: NThierry Reding <treding@nvidia.com>

Documentation/devicetree/bindings/misc/nvidia,tegra20-apbmisc.txt

@@ -10,3 +10,5 @@ Required properties:
        The second entry gives the physical address and length of the
        registers indicating the strapping options.
 
+Optional properties:
+- nvidia,long-ram-code: If present, the RAM code is long (4 bit). If not, short (2 bit).