i2c: nomadik: auto-calculate slave setup time

The Nomadik I2C controller needs to have the slave set-up time
configured based off the clock used to drive the I2C bus block.
Currently this is done with static assignments assuming that the
block is clocked 48MHz which is pretty likely to be bug-prone.
Calculate the SLSU from the equation given in the datasheet
instead.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>