target/nios2: Update helper_eret for shadow registers

When CRS = 0, we restore from estatus; otherwise from sstatus.
Update for the new CRS.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220421151735.31996-56-richard.henderson@linaro.org>

target/nios2/cpu.h

@@ -82,6 +82,7 @@ enum {
     R_FP     = 28,
     R_EA     = 29,
     R_BA     = 30,
+    R_SSTATUS = 30,
     R_RA     = 31,
 };