ARM: mxs: look up timrot clock from device tree

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>