ARM: mxs: remove custom .init_time hook

This patch converts clk-imx2[38] clocksource_of_init compatible init
associated with fsl,imx2[38]-clkctrl. With arch/arm calling
of_clk_init(NULL) from time_init(), we can now also remove custom
.init_time hooks.

Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Mike Turquette <mturquette@linaro.org>
Acked-by: Shawn Guo <shawn.guo@linaro.org>