clk: imx: clk-cpu: Switch to clk_hw based API

Switch the clk_cpu clock registering function to clk_hw based API and add
a macro for clk based legacy. 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>