i.MX2 family: Add clock handling for i.MX27 CPU

Internal clock path handling for the i.MX27 CPU.

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

TODO:
 - Check if the i.MX21 CPU can share the same code.

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