MLK-21828-3 clk-imx8: fix get clk rate issue

MLK-21828-3 clk-imx8: fix get clk rate issue

The SCFW get clock rate API only return u32, but we use a ulong to
get the value. The ulong variable is not initialized, so on stack
it higher 32 bits may not zero, and return invalid rate.

Signed-off-by: Ye Li <ye.li@nxp.com>