The RNG can use HSI48, HSI48/2 or HSI16. Using MSI as 48 MHz clock source for RNG is not possible. The clock configuration in `stmclk_u5.c` activates anyway only the MSIS but not the MSIK which could be used for certain peripherals.
Therefore, this commit
- removes the configuration of MSI as 48 MHz clock for RNG and its selection in `RCC->CCIPR1.ICLKSEL`
- enables HSI48 and selects it for RNG.
The HSI48 will also be used in future for certain peripherals such as USB OTG FS and SDMMC.
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