target/riscv: Add *envcfg* CSRs support

target/riscv: Add *envcfg* CSRs support

The RISC-V privileged specification v1.12 defines few execution
environment configuration CSRs that can be used enable/disable
extensions per privilege levels.

Add the basic support for these CSRs.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Message-Id: <20220303185440.512391-6-atishp@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>