[Rocketchip] Compile the Rocket-chip, write the document.

[Jerryy959]

Rocketchip: https://github.com/BOSC-Hvisor/rocket-chip

[Jerryy959]

In PTW.scala implement file, check this pr chipsalliance/rocket-chip#1624 for pmaPgLevelHomogeneous error.

[Jerryy959]

Learn this discussion chipsalliance/rocket-chip#3168, for understanding the PTW and TLB implement based on rocketchip, which includes two sessions:

• Code Anatomy of PTW and TLB  chipsalliance/rocket-chip#2992, one issue session.
• https://github.com/chipsalliance/rocket-chip/files/9966821/TLB.and.PTW.pdf, a mind map for that.