target/riscv: rvk: add CSR support for Zkr

target/riscv: rvk: add CSR support for Zkr

 - add SEED CSR which must be accessed with a read-write instruction:
   A read-only instruction such as CSRRS/CSRRC with rs1=x0 or CSRRSI/CSRRCI
with uimm=0 will raise an illegal instruction exception.
 - add USEED, SSEED fields for MSECCFG CSR

Co-authored-by: Ruibo Lu <luruibo2000@163.com>
Co-authored-by: Zewen Ye <lustrew@foxmail.com>
Signed-off-by: Weiwei Li <liweiwei@iscas.ac.cn>
Signed-off-by: Junqiang Wang <wangjunqiang@iscas.ac.cn>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20220423023510.30794-13-liweiwei@iscas.ac.cn>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>