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>