clk: imx: imx6sx: use imx_obtain_fixed_clk_hw to simplify code

imx_obtain_fixed_clk_hw could be used to simplify code to replace
__clk_get_hw(of_clk_get_by_name(node, "name"))

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>