[DV infra] Extend the dv_window register to support querying the platform

[engdoreis]

It would be very valuable for software tests if there was a mechanism for querying the underlying platform the test is running on.

This would allow for conditional branching within tests based on the platform, avoiding a combinatorial explosion of binaries.

This could be implemented by extending the dv_window, adding a new field—let's call it HW_ID—which would return a unique but consistent value for each platform:

enum HW_ID {
  FpgaGenesys2 = 0xA,
  SimVerilator = 0x1A,
  SimDv        = 0x2A,
};

[martin-velay]

This could be a new CSR in RO and initialized to the correct ID value by each "HW platform"?