watchdog: Add watchdog_active() routine

Some watchdog may need to check if watchdog is ACTIVE or not, for example in
their suspend/resume hooks.

This patch adds this routine and changes the core drivers to use it.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>