ARM: dts: stm32: add cpufreq support on stm32mp15x

This commit adds cpufreq support on stm32mp15x SOC. STM32 cpufreq uses
operating points V2 bindings (no legacy). Nvmem cells have to be used to
know the chip version and then which OPPs are available. Note that STM32
cpufreq driver is mainly based on "cpufreq-dt" driver.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>