watchdog: add support for the Synopsys DesignWare WDT

The Synopsys DesignWare watchdog is found in several ARM based systems
and provides a choice of 16 timeout periods depending on the clock
input.  The watchdog cannot be disabled once started.

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