hw/misc/sse-cpu-pwrctrl: Implement SSE-300 CPU<N>_PWRCTRL register block

hw/misc/sse-cpu-pwrctrl: Implement SSE-300 CPU<N>_PWRCTRL register block

The SSE-300 has a new register block CPU<N>_PWRCTRL.  There is one
instance of this per CPU in the system (so just one for the SSE-300),
and as well as the usual CIDR/PIDR ID registers it has just one
register, CPUPWRCFG.  This register allows the guest to configure
behaviour of the system in power-down and deep-sleep states.  Since
QEMU does not model those, we make the register a dummy
reads-as-written implementation.

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: 20210219144617.4782-21-peter.maydell@linaro.org