ARM: imx6: use common of_clk_init() call to initialize clocks

ARM: imx6: use common of_clk_init() call to initialize clocks

Instead of explicitly calling clock initialization functions, we can
declare the functions with CLK_OF_DECLARE() and then call common
of_clk_init() to have them invoked properly.

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