i2c-omap: Use I2C interface clock also on OMAP16xx CPUs

Create and use a virtual i2c_ick clock which prevents its parent clock
ARMPER_CK from being stopped during MPU idle whenever i2c_ick is enabled.
Otherwise we end up losing IRQs if ARMPER_CK would otherwise idle.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Juha Yrjola <juha.yrjola@nokia.com>