clk: imx: Add imx_obtain_fixed_clock clk_hw based variant

In order to move to clk_hw based API, imx_obtain_fixed_clock_hw
is added. The end goal here is to have all the clk providers use
the clk_hw based API.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>