clk: imx6sx: Switch to clk_hw based API

Switch the entire clk-imx6sx driver to clk_hw based API. This 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>