clk: imx: scu: add cpu frequency scaling support

On NXP's i.MX SoCs with system controller inside, CPU frequency
scaling can ONLY be done by system controller firmware, and it
can ONLY be requested from secure mode, so Linux kernel has to
call ARM SMC to trap to ARM-Trusted-Firmware to request system
controller firmware to do CPU frequency scaling.

This patch adds i.MX system controller CPU frequency scaling support,
it reuses cpufreq-dt driver and implement the CPU frequency scaling
inside SCU clock driver.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>