[WATCHDOG] davinci watchdog driver

Add watchdog support for TI Davinci DM644x/DM646x processors.

Signed-off-by: Vladimir Barinov <vbarinov@ru.mvista.com>
Signed-off-by: Kevin Hilman <khilman@mvista.com>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>