clock-imx35: Calculate the base clock rate for the IPU unit

The mx3fb driver needs the clock the IPU runs in order to calculate
the divider for the LCD clock. This patch adds the clock rate calculation
routine for the i.MX35 CPU.

Signed-off-by: Juergen Beisert <jbe@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>