The configuration whether a HXTAL is present and what its clock frequency is will be added to Kconfig. Since it is the only GD32V board at the moment, the configuration is added to the Kconfig of the board, but should be moved to a common Kconfig later when more GD32V boards are added.
Setting the `RCU_CTL` register just to the IRC8M bit also removes the IRC8M calibration and trim adjust value in this register. Therefore IRC8M calibration and trim adjust value have to be preserved and the IRC8M has to be set.
