hw/misc/mps2-scc: Implement changes for AN547

hw/misc/mps2-scc: Implement changes for AN547

Implement the minor changes required to the SCC block for AN547 images:
 * CFG2 and CFG5 exist (like AN524)
 * CFG3 is reserved (like AN524)
 * CFG0 bit 1 is CPU_WAIT; we don't implement it, but note this
   in the TODO comment

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20210219144617.4782-40-peter.maydell@linaro.org