R8_PLL_CONFIG and R8_FLASH_CFG are undocumented

[karlp]

The provided startup code both sets and clears bits in the registers R8_PLL_CONFIG (0x4000104B) and R8_FLASH_CFG (0x40001807), but their contents are undocumented in CH583DS1_EN.pdf

See https://github.com/openwch/ch583/blob/main/EVT/EXAM/SRC/StdPeriphDriver/CH58x_sys.c#L29 and https://github.com/openwch/ch583/blob/main/EVT/EXAM/SRC/StdPeriphDriver/CH58x_sys.c#L113

See https://github.com/openwch/ch583/blob/main/EVT/EXAM/SRC/StdPeriphDriver/CH58x_sys.c#L98

The values written to these registers has also changed with library updates, so we need to know what these are for.