clk: mxs: replace imx28 clk_register_clkdev with clock DT lookup

It really becomes a maintenance issue that every time a device needs
to look up (clk_get) a clock we have to patch kernel clock file to call
clk_register_clkdev for that clock.

Since clock DT support which is meant to resolve clock lookup in device
tree is in place, the patch moves imx28 client devices' clock lookup
over to device tree, so that any new lookup to be added at later time
can just get done in DT instead of kernel.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>