Update FireSim with latest AFIs and improvements

[dhand-galois]

Closes #99, #100. May improve #101.

Update the AGFIs in FireSim/README.md. Added some additional information to FireSim/minimal_cloudgfe.md.

In addition, duplicating changelog from https://github.com/DARPA-SSITH-Demonstrators/firesim/pull/2:

This PR brings the following improvements to CloudGFE firesim:

• Improves run-time performance of AFIs by ~50% when using swpkg stand-alone simulation (see https://github.com/DARPA-SSITH-Demonstrators/BESSPIN-CloudGFE/issues/100)
• Update how the UART baud rate gets baked into UARTBridgeModule. Now uses a system-wide fixed baud rate if available
• Tears down simulation environment more completely when simulator exits cleanly
• Passes AWS_DEFAULT_REGION to docker environment, or sets it if not already set
• Bumps chipyard version
  • Chipyard updated to simplify how icenet/iceblk drivers are modified to change their default location
  • Hardcodes the UART baud rate to 115200. UART divisor register becomes read-only
  • Updates bootargs in device tree to use SiFive UART rather than HVC driver
  • Updates pointer to FireSim as a submodule to be our version of FireSim