Documentation: DT: Add entry for ARM SP805-WDT

This patch adds a devicetree binding documentation for ARM's
SP805 WatchDog Timer.

Signed-off-by: Bhupesh Sharma <bhupesh.sharma@freescale.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>