ARM: devicetree: document supported Conexant Digicolor SoC

Of the Digicolor SoCs series only CX92755 is currently supported.
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Olof Johansson <olof@lixom.net>

Documentation/devicetree/bindings/arm/digicolor.txt

+Conexant Digicolor Platforms Device Tree Bindings
+
+Each device tree must specify which Conexant Digicolor SoC it uses.
+Must be the following compatible string:
+
+  cnxt,cx92755