target/riscv: Add MEDELEGH, HEDELEGH csrs for RV32

target/riscv: Add MEDELEGH, HEDELEGH csrs for RV32

Based on privileged spec 1.13, the RV32 needs to implement MEDELEGH
and HEDELEGH for exception codes 32-47 for reserving and exception codes
48-63 for custom use. Add the CSR number though the implementation is
just reading zero and writing ignore. Besides, for accessing HEDELEGH, it
should be controlled by mstateen0 'P1P13' bit.

Signed-off-by: Fea.Wang <fea.wang@sifive.com>
Reviewed-by: Frank Chang <frank.chang@sifive.com>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20240606135454.119186-5-fea.wang@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>