clk: imx6q: Switch to clk_hw based API

Switch the entire clk-imx6q driver to clk_hw based API.
Add imx6q_obtain_fixed_clk_hw helper to clean up the registration
of the clocks that are either found in device tree or are assigned
a fixed zero rate. This switch allows us to move closer to a clear
split between consumer and provider clk APIs.

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