MXC family: Add clock handling

Internal clock path handling for the mxc CPUs.

Changed against the original Freescale code (and against clocklib for example):
 - clock rate is always calculated whenever one ask for the current rate
   (means struct clk has no more a member called "rate"). So switching the PLL
   base frequency will propagate immediately to all other clocks that are
   depending on this frequency.

Signed-off-by: Juergen Beisert <j.beisert@pengutronix.de>