target/nios2: Split control registers away from general registers

target/nios2: Split control registers away from general registers

Place the control registers into their own array, env->ctrl[].

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220421151735.31996-23-richard.henderson@linaro.org>