dts: arm: st: stm32mp2_m33.dtsi: add HSE and HSI fixed clock

Add declarations for High-Speed External (HSE) and High-Speed Internal
(HSI) clocks.
These clocks, based on oscillators, can be used to generate the system
clocks.

Signed-off-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>