[WATCHDOG] Add support for the Avionic Design Xanthos watchdog timer.

This patch adds support for the watchdog timer on Avionic Design Xanthos
boards.

Signed-off-by: Thierry Reding <thierry.reding@avionic-design.de>
Acked-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>