hw/misc/iotkit-sysctl: Remove is_sse200 flag

hw/misc/iotkit-sysctl: Remove is_sse200 flag

Remove the is_sse200 flag in favour of just directly testing the new
sse_version field.

Since some of these registers exist in the SSE-300 but some do not or
have different behaviour, we expand out the if() statements in the
read and write functions into switch()es, so we have an easy place to
put SSE-300 specific behaviour.

(Until we do add the SSE-300 behaviour, the thing preventing us
reaching the "unreachable" default cases is that armsse.c doesn't
yet pass us an ARMSSE_SSE300 version.)

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: 20210219144617.4782-7-peter.maydell@linaro.org