watchdog: Adding support for ARM Primecell SP805 Watchdog

Technical Reference Manual can be found at:
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0270b/DDI0270.pdf



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