i2c: add support for the Digicolor I2C controller

The CX92755 is an SoC in the Conexant Digicolor series. The devicetree binding
document describes the I2C controller on the CX92755 SoC, that is also shared
by some other SoCs in the Digicolor series. The driver adds support.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
[wsa: fixed spaces around operators]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>