hw/ssi/pl022: Allow use as embedded-struct device

hw/ssi/pl022: Allow use as embedded-struct device

Create a new include file for the pl022's device struct,
type macros, etc, so that it can be instantiated using
the "embedded struct" coding style.

While we're adding the new file to MAINTAINERS, add
also the .c file, which was missing an entry.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180820141116.9118-16-peter.maydell@linaro.org
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>