Fix calculation of I2C clock for some 86xx chips

Some 86xx chips use CCB as the base clock for the I2C, and others used CCB/2.
There is no pattern that can be used to determine which chips use which
frequency, so the only way to determine is to look up the actual SOC
designation and use the right value for that SOC.

Signed-off-by: Timur Tabi <timur@freescale.com>