ARM: mxs: look up timrot clock from device tree

Change call clk_get_sys() to of_clk_get() to look up timrot clock from
device tree, so that the clk_register_clkdev() call for timrot can be
saved in clock driver.

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

arch/arm/boot/dts/imx28.dtsi

@@ -838,6 +838,7 @@
 				compatible = "fsl,imx28-timrot", "fsl,timrot";
 				reg = <0x80068000 0x2000>;
 				interrupts = <48 49 50 51>;
+				clocks = <&clks 26>;
 			};
 
 			auart0: serial@8006a000 {