ARM: imx: add clocking support code for the IMX50 SoC

Add code to support the specific clock tree of the Freescale IMX50 SoC.
It can use much of the common IMX51/IMX53 clocking code.

Signed-off-by: Greg Ungerer <gerg@uclinux.org>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>