Generic watchdog device model implementation as per ARM SBSA v6.0

Signed-off-by: Shashi Mallela <shashi.mallela@linaro.org>
Message-id: 20201027015927.29495-2-shashi.mallela@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>