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.
`CONFIG_BOARD_HAS_HXTAL` is used to indicate that the board has an HXTAL connected. If the HXTAL is present, it is used as PLL clock source. But if the HXTAL is not present, the half IRC8M clock should be used as PLL clock source and must not be disabled at the end of clock settings. Using IRC8M clock as PLL clock source also requires another PLL multiplication factor.
`CLOCK_HXTAL` is a value and not a flag, so that shifting to the left changes anything in the register but does not set the PLLSEL bit. `RCU_CFG0_PLLSEL_Msk` has to be used instead to set the PLLSEL bit.
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.
This adds support for the gd32v class devices from Gigadevice. The
gd32vf103 contains an 108 MHz RISC-V core with similar peripherals as
the stm32f1 devices
Co-authored-by: Benjamin Valentin <benjamin.valentin@ml-pa.com>
