watchdog: Add Broadcom BCM2708 watchdog timer driver

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Wim Van Sebroeck <wim@iguana.be>
Cc: linux-rpi-kernel@lists.infradead.org
Cc: linux-watchdog@vger.kernel.org