The MPS2 SCC device doesn't have any documentation of its properties;
add a "QEMU interface" format comment describing them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210504120912.23094-2-peter.maydell@linaro.org