Use 512k base memory for CS8220 chipset if memory larger than 640k, fix some BIOS not recognizing extended memory.

Summary

CS8220 supports 1024KB memory to spilt into 512KB base + 512KB extended memory only, and if using 640KB base + 384KB extended memory spilt, some BIOSes will not recognize extended memory. This commit fixes 768KB, 896KB and 1024KB memory configuration for machines using CS8220 chipset.

Checklist

• Closes Memory size issue with C&T PC/AT chipset #6444
• I have tested my changes locally and validated that the functionality works as intended
• I have discussed this with core contributors already
• This pull request requires changes to the ROM set
  • I have opened a roms pull request - https://github.com/86Box/roms/pull/changeme/

References

https://www.os2museum.com/wp/another-strange-286-board/