i2c: vt8500: Add support for I2C bus on Wondermedia SoCs

This patch adds support for the I2C bus controllers found on Wondermedia
8xxx-series SoCs. Only master-mode is supported.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
[wsa: fixed one macro to shift 8 instead of 16]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>