hw/misc/iotkit-sysctl: Implement dummy version of SSE-300 PWRCTRL register

hw/misc/iotkit-sysctl: Implement dummy version of SSE-300 PWRCTRL register

The SSE-300 has a new PWRCTRL register at offset 0x1fc (previously
reserved). This register controls accessibility of some registers
in the Power Policy Units (PPUs). Since QEMU doesn't implement
the PPUs, we don't need to implement any real behaviour for this
register, so we just handle the UNLOCK bit which controls whether
writes to the register itself are permitted and otherwise make it
be reads-as-written.

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: 20210219144617.4782-17-peter.maydell@linaro.org