MLK-14957 clk: imx8qm/qxp: delay the clk driver initialization

MLK-14957 clk: imx8qm/qxp: delay the clk driver initialization

Using CLK_OF_DECLARE will register clks at very early stage
that AP-SC communication still not ready.

MU driver is registered by early_initcall, so using core_initcall
to delay the registeration of clk driver is ok.

Move channel open function into probe to avoid core_init sequence
not in our expected order. If MU not ready, return defer probe
for clk driver.

Later when we switch to use interrupt driver AP-SC communication,
we could also return defer probe in clk probe function to wait
until AP-SC communication ready.

Signed-off-by: Peng Fan <peng.fan@nxp.com>