imx6: clock: improve calculations to get the PLL video rate

imx6: clock: improve calculations to get the PLL video rate

During some tests to check the pixel clock rate in the transition from
U-Boot to the Linux kernel, I noticed that with the same configuration
of the registers the debug messages reported different rates.

The same Linux kernel calculations are now used to get the PLL video
rate.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Michael Trimarchi <michael@amarulasolutions.com>