hw/ssi/pl022: Set up reset function in class init

hw/ssi/pl022: Set up reset function in class init

Currently the PL022 calls pl022_reset() from its class init
function. Make it register a DeviceState reset method instead,
so that we reset the device on system reset.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180820141116.9118-17-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>