target/riscv: Allow debugger to access seed CSR

At present seed CSR is not reported in the CSR XML hence gdb cannot
access it.

Fix it by adding a debugger check in its predicate() routine.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Weiwei Li <liweiwei@iscas.ac.cn>
Reviewed-by: LIU Zhiwei <zhiwei_liu@linux.alibaba.com>
Message-ID: <20230228104035.1879882-15-bmeng@tinylab.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

target/riscv/csr.c

@@ -459,6 +459,10 @@ static RISCVException seed(CPURISCVState *env, int csrno)
     }
 
 #if !defined(CONFIG_USER_ONLY)
+    if (env->debugger) {
+        return RISCV_EXCP_NONE;
+    }
+
     /*
      * With a CSR read-write instruction:
      * 1) The seed CSR is always available in machine mode as normal.