clk: imx: Remove __init for imx_obtain_fixed_clk_hw() API

Some of i.MX SoCs' clock driver will use platform driver model,
and they need to call imx_obtain_fixed_clk_hw() API, so
imx_obtain_fixed_clk_hw() API should NOT be in .init section.

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