include: Add defines for [m|h|s]envcfg CSRs

The latest RISC-V privileged specification introduces xenvcfg CSRs
to enable/disable certain features/extensions for lower privilege
modes. This patch adds defines for these new [m|h|s]envcfg CSRs.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Xiang W <wxjstz@126.com>