CVA6 CSR ACCESS TEST PR

Created by: spidugu444

Hi @JeanRochCoulon, as per you suggestion, I simplify PR by addressing only MSCRATCH CSR. Gitlab CI is running fine for the same, please find the GitLab CI here.

Highlights:

1. Added script to run CSR ACCESS tests.
2. Added new test list file for CSR ACCESS tests.
3. Added new test directory for CSRs in tests/custom
4. Added new directory for CSR yaml files in tests/custom
5. Removed riscv-isa-sim patch, which is used to generate ecall to terminate the test.
6. Added DV plan for MSCRATCH CSR.

please let me know if any changes needed.

Thanks