Add tests for Sspmp extension

[ybc-alkaid]

This PR introduces basic tests for the RISC-V S-level Physical Memory Protection (SPMP) extension.

Here is a pre-built PDF of SPMP:
https://github.com/riscv/riscv-spmp/blob/main/rv-spmp-spec.pdf

Here is a pre-built PDF of riscv-privileged manual with SPMP:
https://github.com/riscv/riscv-spmp/releases/download/v0.8.18/riscv-privileged-with-spmp.pdf

Here is the pointer to the SPMP specification:
riscv/riscv-isa-manual#2573

Here is the Spike implementation supporting the tests:
riscv-software-src/riscv-isa-sim#2244

Here are the test logs:
https://drive.google.com/drive/folders/1MT65PQ7_RgEeZ8O90XkNR9AzX2AzLgIe?usp=drive_link

[jordancarlin]

Hi @ybc-alkaid. Thanks for your contribution. The riscv-arch-test repository recently migrated to ACT4 (version 4 of the test suite) and is no longer adding new extensions to the old 3.0 framework. Please review the guidelines in the Developer Guide for instructions on writing tests and coverpoints for the act4 framework. Unfortunately, there is likely to be some significant work involved in porting these tests.

Please reach out with any questions or documentation suggestions and let us know if you need help porting the tests.