ARM: dts: stm32mp1: use OPP information for PLL1 settings in SPL

This patch allows to switch the CPU frequency to 800MHz on the
ST Microelectronics board (DK1/DK2 and EV1) or dh electronics SOM
using the STM32MP15x SOC and when it is supported by the HW
(for STM32MP15xD and STM32MP15xF).

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