clock-imx35: correct arm and ahb clock calculation

According to the Datasheet:
"i.MX35 (MCIMX35) Multimedia Applications Processor Reference Manual,
Rev. 2" "Figure 14-24. Clock Control And Gating"
change the result of get_rate_ahb based on the frequency returned
by get_rate_arm to calculate the proper rate.

Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Signed-off-by: Michael Grzeschik <m.grzeschik@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>