hw/timer/pl031: Allow use as an embedded-struct device

hw/timer/pl031: Allow use as an embedded-struct device

Create a new include file for the pl031's device struct,
type macros, etc, so that it can be instantiated using
the "embedded struct" coding style.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>