drivers: add stm32 CPU DVFS driver

drivers/cpu_opp.c implements dynamic voltage and frequency
scaling for the CPU.
It is used at boot time to set an higher operating point than
the one used to boot.

drivers: add stm32 CPU DVFS driver

drivers/cpu_opp.c implements dynamic voltage and frequency
scaling for the CPU.
It is used at boot time to set an higher operating point than
the one used to boot.
It will be used by the SCMI performance service.

Signed-off-by: Pascal Paillet <p.paillet@foss.st.com>
Reviewed-by: Etienne Carriere <etienne.carriere@foss.st.com>

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